Enable HW dithering in a few key places (#4541)

* Enable HW dithering on supported devices (Clara HD, Forma; Oasis 2, PW4)
  * FileManager and co. (where appropriate, i.e., when covers are shown)
  * Book Status
  * Reader, where appropriate:
    * CRe: on pages whith image content (for over 7.5% of the screen area, should hopefully leave stuff like bullet points or small scene breaks alone).
    * Other engines: on user-request (in the gear tab of the bottom menu), via the new "Dithering" knob (will only appear on supported devices).
  * ScreenSaver
  * ImageViewer
* Minimize repaints when flash_ui is enabled (by, almost everywhere, only repainting the flashing element, and not the toplevel window which hosts it).
  (The first pass of this involved fixing a few Button instances whose show_parent was wrong, in particular, chevrons in the FM & TopMenu).
* Hunted down a few redundant repaints (unneeded setDirty("all") calls),
  either by switching the widget to nil when only a refresh was needed, and not a repaint,
  or by passing the appropritate widget to setDirty.
  (Note to self: Enable *verbose* debugging to catch broken setDirty calls via its post guard).
  There were also a few instances of 'em right behind a widget close.
* Don't repaint the underlying widget when initially showing TopMenu & ConfigDialog.
  We unfortunately do need to do it when switching tabs, because of their variable heights.
* On Kobo, disabled the extra and completely useless full refresh before suspend/reboot/poweroff, as well as on resume. No more double refreshes!
* Fix another debug guard in Kobo sysfs_light
* Switch ImageWidget & ImageViewer mostly to "ui" updates, which will be better suited to image content pretty much everywhere, REAGL or not.

PS: (Almost 💯 commits! :D)

frontend/ui/widget/menu.lua

@@ -403,19 +403,21 @@ function MenuItem:onTapSelect(arg, ges)
         coroutine.resume(co)
     else
         self[1].invert = true
-        UIManager:setDirty(self.show_parent, function()
+        UIManager:widgetRepaint(self[1], self[1].dimen.x, self[1].dimen.y)
+        UIManager:setDirty(nil, function()
             return "fast", self[1].dimen
         end)
         UIManager:tickAfterNext(function()
-            self[1].invert = false
-            UIManager:setDirty(self.show_parent, function()
-                return "ui", self[1].dimen
-            end)
             logger.dbg("creating coroutine for menu select")
             local co = coroutine.create(function()
                 self.menu:onMenuSelect(self.table, pos)
             end)
             coroutine.resume(co)
+            self[1].invert = false
+            --UIManager:widgetRepaint(self[1], self[1].dimen.x, self[1].dimen.y)
+            UIManager:setDirty(self.show_parent, function()
+                return "ui", self[1].dimen
+            end)
         end)
     end
     return true
@@ -427,15 +429,17 @@ function MenuItem:onHoldSelect(arg, ges)
         self.menu:onMenuHold(self.table, pos)
     else
         self[1].invert = true
-        UIManager:setDirty(self.show_parent, function()
+        UIManager:widgetRepaint(self[1], self[1].dimen.x, self[1].dimen.y)
+        UIManager:setDirty(nil, function()
             return "fast", self[1].dimen
         end)
         UIManager:tickAfterNext(function()
+            self.menu:onMenuHold(self.table, pos)
             self[1].invert = false
+            --UIManager:widgetRepaint(self[1], self[1].dimen.x, self[1].dimen.y)
             UIManager:setDirty(self.show_parent, function()
                 return "ui", self[1].dimen
             end)
-            self.menu:onMenuHold(self.table, pos)
         end)
     end
     return true
@@ -587,25 +591,25 @@ function Menu:init()
         icon = "resources/icons/appbar.chevron.left.png",
         callback = function() self:onPrevPage() end,
         bordersize = 0,
-        show_parent = self,
+        show_parent = self.show_parent,
     }
     self.page_info_right_chev = Button:new{
         icon = "resources/icons/appbar.chevron.right.png",
         callback = function() self:onNextPage() end,
         bordersize = 0,
-        show_parent = self,
+        show_parent = self.show_parent,
     }
     self.page_info_first_chev = Button:new{
         icon = "resources/icons/appbar.chevron.first.png",
         callback = function() self:onFirstPage() end,
         bordersize = 0,
-        show_parent = self,
+        show_parent = self.show_parent,
     }
     self.page_info_last_chev = Button:new{
         icon = "resources/icons/appbar.chevron.last.png",
         callback = function() self:onLastPage() end,
         bordersize = 0,
-        show_parent = self,
+        show_parent = self.show_parent,
     }
     self.page_info_spacer = HorizontalSpan:new{
         width = Screen:scaleBySize(32),
@@ -699,7 +703,7 @@ function Menu:init()
             if self.onReturn then self:onReturn() end
         end,
         bordersize = 0,
-        show_parent = self,
+        show_parent = self.show_parent,
         readonly = self.return_arrow_propagation,
     }
     self.page_return_arrow:hide()
@@ -953,7 +957,7 @@ function Menu:updateItems(select_number)
         self.path_text.text = self:truncatePath(self.path)
     end
 
-    UIManager:setDirty("all", function()
+    UIManager:setDirty(self.show_parent, function()
         local refresh_dimen =
             old_dimen and old_dimen:combine(self.dimen)
             or self.dimen