panel: Add more options to Editor, turn it into a proper dialog

This is a bit of a mini UI overhaul, just making the panel editor look more
like an actual dialog (By using Gtk.Dialog, whodathunkit) and *slightly*
more HIG3 (GNOME) conformant. Not completely, because it looks like crap.

This commit also brings in the gsettings based options, i.e. size, location
and gnome-panel-theme-integration.