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.

Recipe

@@ -322,6 +322,12 @@ pageant  : [X] uxpgnt uxagentc pageant sshrsa sshpubk sshdes sshbn sshmd5
 PuTTY    : [MX] osxmain OSXTERM OSXMISC CHARSET U_BE_ALL NONSSH UXSSH
          + ux_x11 uxpty uxsignal testback putty.icns info.plist
 
+ptermapp : [XT] GTKTERM uxmisc misc ldisc settings uxpty uxsel BE_NONE uxstore
+         + uxsignal CHARSET cmdline uxpterm version time xpmpterm xpmptcfg
+         + nogss gtkapp
+puttyapp : [XT] GTKTERM uxmisc misc ldisc settings uxsel U_BE_ALL uxstore
+         + uxsignal CHARSET uxputty NONSSH UXSSH UXMISC ux_x11 xpmputty
+         + xpmpucfg gtkapp
 fuzzterm : [UT] UXTERM CHARSET misc uxmisc uxucs fuzzterm time settings
 	 + uxstore be_none
 testbn   : [UT] testbn sshbn misc conf tree234 uxmisc