In the external UI mode we may end up not needing a connection to the
display server at all.

This splits the parsing of Gtk+ command line arguments from connection
to the display. This also fixes the --help output to actually include
the Gtk+ arguments (such as --display).

By now nobody should be using this. Keep the code around for a little
longer just in case anybody still uses this.

The libnm-glib support also serves as an example how do we build two
different versions of the properties plugin. We'll soon be in a similar
situation with Gtk 4.0. (sigh.) Just don't drop it yet.

!4