main: Move KeybindingMode into Shell

Having the definition in C instead of Javascript allows sharing
the corresponding header with gnome-settings-daemon.

https://bugzilla.gnome.org/show_bug.cgi?id=643111

js/ui/lookingGlass.js

@@ -1133,7 +1133,7 @@ const LookingGlass = new Lang.Class({
         if (this._open)
             return;
 
-        if (!Main.pushModal(this._entry, { keybindingMode: Main.KeybindingMode.LOOKING_GLASS }))
+        if (!Main.pushModal(this._entry, { keybindingMode: Shell.KeyBindingMode.LOOKING_GLASS }))
             return;
 
         this._notebook.selectIndex(0);