background: use GFiles instead of filenames

We want to use GResources for system backgrounds, so move this to a
GFile.

https://bugzilla.gnome.org/show_bug.cgi?id=736936

[endlessm/eos-shell#3747]
Merge pull request #17 from endlessm/shell/3747

background: use GFiles instead of filenames
Merge branch 'master' for Release_2.1.7-rc5