LibGUI: Handle Action keyboard shortcuts in Widget keydown

Widgets can now prevent shortcut Actions from being called, which
allows text input keydown handlers to override single key shortcuts.

Userland/Libraries/LibGUI/Window.h

@@ -234,6 +234,8 @@ public:
 
     void set_always_on_top(bool always_on_top = true);
 
+    void propagate_shortcuts_up_to_application(KeyEvent& event, Widget* widget);
+
 protected:
     Window(Core::Object* parent = nullptr);
     virtual void wm_event(WMEvent&);