background: Use a stack with three views for the chooser dialog

Instead of using a GtkGrid and GtkBox to mimic what GtkStack and
GtkStackSwitcher already does for us, let's just use the latter. As a
result each source now has its separate GtkIconView. The pictures
source has a nested GtkStack to switch between the 'empty' box and the
view, which should simplify size-related issues.

https://bugzilla.gnome.org/show_bug.cgi?id=736366