We often use rows representing different things in the same list
box. In these cases it's often desirable to have rows emitting their
own activation signals instead of having a single handler for the
whole list box that dispatches according to the activated row.
https://bugzilla.gnome.org/show_bug.cgi?id=785949
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
