LibGUI+WindowServer: Make it possible to have checkable GActions.

They show up as checkable GButtons in GToolBar, and with (or without) check
marks in menus.

There are a bunch of places to make use of this. This patch only takes
advantage of it in the FileManager for the view type actions.

Servers/WindowServer/WSAPITypes.h

@@ -246,6 +246,8 @@ struct WSAPI_ClientMessage {
             char shortcut_text[32];
             int shortcut_text_length;
             bool enabled;
+            bool checkable;
+            bool checked;
             WSAPI_Point position;
             bool top_anchored;
         } menu;