background: Remove empty property getters and setters

https://bugzilla.gnome.org/show_bug.cgi?id=731713
background: Rearrange the GObject boilerplate

... so that it is possible to override virtual functions and change
the order of the object creation and destruction process without having
to write a bunch of prototypes. Having the class_init below the others
helps us do that.

https://bugzilla.gnome.org/show_bug.cgi?id=731713
background: Use higher resolution thumbnails on HiDpi displays

https://bugzilla.gnome.org/show_bug.cgi?id=731713
background: Remove unnecessary initialization

The entire private struct is initialized to 0 by GObject, so there is
no need to do that again.

https://bugzilla.gnome.org/show_bug.cgi?id=731713