Remove unused "backgrounds-liststore" object from .ui file, and
add remove button.
Make remove button sensitive for files that were copied to the cache,
and implement remove itself.
Remember needs_download when copying CcBackgroundItem objects.
Make "Add" button unsensitive in chooser when background was
already added to the list, to avoid duplicates.
Ellipsise background label.
Now we'd just need gnome-bg to make the jump.
This also fixes unique IDs for the wallpapers source, as
multiple backgrounds can be defined in a single XML file.
It now is a full GObject, will be able to track the original
XML file for a wallpaper, as well as which values are set in it.
This will allow us to restore wallpapers properly, and only offer
editing options when those aren't already hard-coded inside the
item.
