Port conversation list to Gtk.ListBox [$60]

This would:

* Fix the awkward :selected and :focus highlighting issues such as the text and icons being inconsistent in their coloring.
* Allow us to use gtk.revealer to animate adding and removing messages
* Make the layout code actually legible
* Allow easily styling with css classes (like .dim-label) instead of manually drawing in cairo
* Allow us to use native gtk signals for selection, activated etc instead of relying on gdk mouse events and manual coordinates