New front end to PuTTY/pterm, as a GtkApplication.

New front end to PuTTY/pterm, as a GtkApplication.

When it's finished, this will be the backbone of the OS X GTK port:
using a GtkApplication automatically gives us a properly OS X
integrated menu bar.

Using this source file in place of gtkmain.c turns the usual Unix
single-session-per-process PuTTY or pterm into the multi-session-per-
process OS X style one.

Things like Duplicate Session can be done much more simply here - we
just grab the Conf * from the source window and launch a new window
using it, with no fiddly interprocess work needed.

This is still experimental and has a lot of holes, but it's usable
enough to test and improve.