Improve appearance under Unity: Closes bug #727624

Toolbar improvments, primarily by removing label outright and letting
icons do all the work. Also, from some user request, have tried
moving icons to right side of toolbar, where they're closer to
the conversation viewer.

This does not fix dragging the window by the toolbar (that's fixed
in 3.10, bug #709914) or the title/subtitle in the GtkHeaderBar
being too close. We might consider using a custom title widget.