The ability to choose solid colored backgrounds has been removed at
least 4 years ago, so let's remove the code for it too.
Furthermore, we slightly refactor some code to make it easier to
understand and fit better to the usual style.
Because the CcBackgroundPaintable is now reusable, it can be used inside
the preview. This allows removing lots of code, since the drawing is now
done by the paintable. Because the scale factor was previously not
handled properly, this change makes the preview much sharper on scale
factors > 1.
Fixes#704
Since 7cef6dc5, the background previews are rendered at the full
resolution. However, this is expensive and causes considerable lag,
especially for resizes. Since then, the thumbnail size has doubled to
256 pixels, making the previews sharper even at larger sizes.
So, to improve performance, let's effectively revert 7cef6dc5. The
revert is not possible without disabling frame = 0 retrieval, since that
is broken for non-slideshow backgrounds and was not used before anyways
with force_size = TRUE.
Related to #674
Related to #704
Related to #2448
This is a dedicate class to generate the preview. Not only
it renders the pixbuf of the wallpaper, but it also adds
lock screen or desktop frames on top of it. It also monitors
the time to keep the lock screen preview label always updated.
