* 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)
* Prevent open screensaver more than one in the same time
* Add option to enable/disable background in message screensaver
* fallback to random images if this book cover is excluded feature + fix Disabled (leave screen/page as it is)
* Autostretch disable by default, more fallback options, leave screen as it is
