gtk: implement -full-screen

Aiming for GTK as replacement for SDL, a feature like -full-screen should also
be implemented.

Bringing the window into full-screen mode is done by activating the "Fullscreen"
menu item. This is done after showing the windows to make the cursor and menu
hidden.

v2: drop -no-frame implementation, use booleans instead of ints and ensure
    consistency between ui state and menu.

Signed-off-by: Peter Wu <lekensteyn@gmail.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/vl.c b/vl.c
index 180fdde..169c807 100644
--- a/vl.c
+++ b/vl.c
@@ -4348,7 +4348,7 @@
 #endif
 #if defined(CONFIG_GTK)
     case DT_GTK:
-        gtk_display_init(ds);
+        gtk_display_init(ds, full_screen);
         break;
 #endif
     default: