Support DBus activation
Updates the client application to be DBus activatable.
This branch adds a DBus service file for Geary, updates the desktop file to enable desktop activation from the shell, converts to using the standard GApplication command line option parsing framework (so Geary now supports standard GLib and GTK command line options), deprecates the
--hidden command line option in favour of
--gapplication-service, updates the proforma autostart desktop to use this, updates the user's installed autostart desktop file if needed.