Button: Better handling of translucent MovableContainer (#7223)

* DictQuickLookup: Preserve alpha when switching dict, and scrolling
inside a dict.

* Start moving the NumberPicker alpha hack to Button itself

This makes handling flash_ui easier and saner, avoiding flickering.

* Handle the transparency hack entirely from within Button

* Murder the now unnecessary NumberPicker update_callback hack

* Tweak comments

* And the Button handling made that redundant, too

* Squish debug print

* More comment tweaks

* Reset transparency on scrolling instead of rpeserving it

* Reset alpha when switching dictionaries

* Simplify the pre/post callbakc transparency state handling

And explain why we need to care.

* Give a named reference to ButtonDialog's MovableContainer, so the Button
alpha hack behaves with it

* Document the "self.movable" convention

* Amend that comment a bit

e.g., we don't care much about MultiConfirmBox'w MpvableContainer, as
any button action will close it.

* And make SkimTo's MovableContainer accessible so that Button can grok
that it's translucent

frontend/ui/widget/scrolltextwidget.lua

@@ -150,9 +150,17 @@ function ScrollTextWidget:updateScrollBar(is_partial)
         if is_partial then
             refreshfunc = "partial"
         end
-        UIManager:setDirty(self.dialog, function()
-            return refreshfunc, self.dimen
-        end)
+        -- Reset transparency if the dialog's MovableContainer is currently translucent...
+        if is_partial and self.dialog.movable and self.dialog.movable.alpha then
+            self.dialog.movable.alpha = nil
+            UIManager:setDirty(self.dialog, function()
+                return refreshfunc, self.dialog.movable.dimen
+            end)
+        else
+            UIManager:setDirty(self.dialog, function()
+                return refreshfunc, self.dimen
+            end)
+        end
         if self.scroll_callback then
             self.scroll_callback(low, high)
         end