To make the CcBackgroundPaintable fully reusable, it has to know the
desired thumbnail size and the thumbnail factory. We add a width and
height property, and modify the thumbnail retrieval of CcBackgroundItem
to use the scale factor now.
This also allows us to greatly simplify the background sources, so that
they now only do one thing: provide CcBackgroundItems from different
sources.
We keep the fixed thumbnail height and width for the chooser, which
makes more sense now that it is in the chooser code instead of the
background source code.
BgRecentSource is a background source implementation that
keeps track of recently added backgrounds.
The backgrounds are copied to $(local data dir)/backgrounds,
which is monitored. When copying files to this folder, the
current time is added as prefix to avoid filename collisions.
