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
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.
