Background xml files can contain multiple background definitions, each
with a name and (dark) filename. Each of them is individually added as a
CcBackgroundItem. Duplicates are checked based on an id, which is formed
by concatenating the xml filename and the background name.
However, the name of the background is not used at the moment, so could
be not unique, making the ids for multiple backgrounds in one xml also
not unique, resulting in missing backgrounds. This has not been an issue
much anymore, because gnome-backgrounds switched to separate xml files.
Nevertheless, improve this by also adding the actual image filename to
the hash id, as suggested in #1115.
Closes#1115
Related gnome-backgrounds#10
d87587946a
