messageTray: Summary items should be square and have spacing

messageTray: Summary items should be square and have spacing

Each summary icon is 48x48 with a padding of 6px on each side. Thus,
each summary item is 60px wide. Therefore the summary mode should be
60px high instead of 72px.

Changed the tray actor to use a ClutterBinLayout so that it honors the
y-expand property of its children.

https://bugzilla.gnome.org/show_bug.cgi?id=682248