So that the Media sharing dialogue in the Sharing panel isn't too high.
GtkListBox doesn't implement GtkScrollable so it needs to be added to a GtkViewport. Instead of having to do this in all .ui files, add a new helper function to setup the widget tree correctly. https://bugzilla.gnome.org/show_bug.cgi?id=732175
Add helpers for GtkListBox sizing, and setting separators. https://bugzilla.gnome.org/show_bug.cgi?id=732106