Make sure item_added_id is set to zero only when the idle source id is
actually removed. This ensures that the source is removed during finalize
if it still exists when the object is destroyed.
The file is in $incdir/gsettings-desktop-schemas/gdesktop-enums.h,
and pkg-config returns -I$incdir/gsettings-desktop-schemas, so the
right way is to #include <gdesktop-enums.h> which is what other modules
do.
https://bugzilla.gnome.org/show_bug.cgi?id=642369
If an xml file references a wallpaper that doesn't exist
(as seen by installing the desktop-backgrounds-basic package
on Fedora), then just don't add it to the list of wallpapers.
Reported by Matthias Clasen
