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/checkbutton.lua

@@ -96,14 +96,16 @@ function CheckButton:onTapCheckButton()
         if G_reader_settings:isFalse("flash_ui") then
             self.callback()
         else
-            self.invert = true
-            UIManager:setDirty(self.show_parent, function()
+            self[1].invert = true
+            UIManager:widgetRepaint(self[1], self.dimen.x, self.dimen.y)
+            UIManager:setDirty(nil, function()
                 return "fast", self.dimen
             end)
             UIManager:tickAfterNext(function()
                 self.callback()
-                self.invert = false
-                UIManager:setDirty(self.show_parent, function()
+                self[1].invert = false
+                UIManager:widgetRepaint(self[1], self.dimen.x, self.dimen.y)
+                UIManager:setDirty(nil, function()
                     return "fast", self.dimen
                 end)
             end)