Create OS X application bundles for PuTTY and pterm.

This commit adds two .plist files, which go in the app bundles; two
.bundle files, which are input to gtk-mac-bundler and explain to it
how to _create_ the bundles; and a piece of manual addition to
Makefile.am that actually runs gtk-mac-bundler after building the
gtkapp.c based binaries and the OSX launcher. The latter is
conditionalised on configuring --with-quartz (unlike the binaries
themselves, which you can build on other platforms too, though they
won't do much that's useful).

unix/gtkapp.c

@@ -7,6 +7,22 @@
  * it's fully working.)
  */
 
+/*
+
+To build on OS X, you will need a build environment with GTK 3 and
+gtk-mac-bundler, and also Halibut on the path (to build the man pages,
+without which the standard Makefile will complain). Then, from a clean
+checkout, do this:
+
+./mkfiles.pl -U --with-quartz
+make -C icons icns
+make -C doc
+make
+
+and you should get unix/PuTTY.app and unix/PTerm.app as output.
+
+*/
+
 #include <assert.h>
 #include <stdlib.h>