Deal with unknown flags from ClutterEvent.get_state()

When we get a ClutterModifierType from Clutter, it might contain
bits not in the enumeration. See bug 59771 for a similar problem
with GdkModifierType.

Add a wrapper Shell.get_event_state() around clutter_event_get_state()
to mask these bits out and only return approved bits.

https://bugzilla.gnome.org/show_bug.cgi?id=597735

src/shell-global.h

@@ -79,6 +79,8 @@ GdkRectangle *shell_global_get_focus_monitor   (ShellGlobal  *global);
 
 GdkModifierType shell_global_get_modifier_keys (ShellGlobal *global);
 
+ClutterModifierType shell_get_event_state (ClutterEvent *event);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */