The first input source gets activated by default when users log in but
we don't currently allow users to set this (other than the awkward
remove all input sources and add them again in the right order).
Allowing user re-ordering of the input source list nicely allows users
to get the default input source they want.
https://bugzilla.gnome.org/show_bug.cgi?id=743400
Currently it looks a bit unpolished that the add/remove buttons on the
other side of the toolbar look linked while this pair doesn't. Note
that the input source settings button isn't always visible but in that
case the layout button will still show up correctly as non-linked.
https://bugzilla.gnome.org/show_bug.cgi?id=743400
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
