gtk: Move GLib, Pango and Cairo code to separate systems.

gtk: Move GLib, Pango and Cairo code to separate systems.

This separation allows the GIO interface (and Pango and Cairo) to work
without a DISPLAY.  The new (glib) package calls g_main_loop_run with
Scheme attached as a GSource.  Thus gtk-thread is now glib-thread, and
stop-gtk-thread is now stop-glib-thread.

The Cairo system depends on Pango and thus GLib only because Cairo
seems useless without Pango.  Thus there is no need for a Pango-Cairo
system.

The Gtk system just calls gtk_init_check, not gtk_main.  This appears
sufficient to get DISPLAY attached to the already running GMainLoop.
Gtk-thread-running? is now gtk-initialized?.