Move from optparse to argparse

That's a thing that people do...
Drop support for pygtk and Gtk2

The time has come.
Port to GtkApplication

Removes some of the "features" provided by the dbus service but these
could probably be added back.
Merge pull request #12 from leighman/gtk3

Drop broken old code for supporting Gtk+ 2, PyGtk, EggTrayIcon