window: Support GTK+'s hide-titlebar-when-maximized hint

For maximized windows, titlebars cannot be used to reposition or
scale the window, so if an application does not use it to convey
useful information (other than the application name), the screen
space occupied by titlebars could be put to better use.
To account for this use case, a setting for requesting that windows'
titlebars should be hidden during maximization has been added to
GTK+, add support for this in the window manager.

https://bugzilla.gnome.org/show_bug.cgi?id=665617

src/core/window-private.h

@@ -105,6 +105,7 @@ struct _MetaWindow
   char *dbus_unique_name;
   char *dbus_object_path;
 
+  int hide_titlebar_when_maximized;
   int net_wm_pid;
   
   Window xtransient_for;