Undefine the value before overriding it to avoid the following warnings:
../panels/background/cc-background-chooser.c:22: warning: "G_LOG_DOMAIN" redefined
The BG_RECENT_SOURCE (user_data) call was done before it was checked if is was
cancelled. At this point user_data is pointing to an object that has been deleted.
Fixes#729
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.