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/modalDialog.js

@@ -37,7 +37,7 @@ const ModalDialog = new Lang.Class({
         params = Params.parse(params, { shellReactive: false,
                                         styleClass: null,
                                         parentActor: Main.uiGroup,
-                                        keybindingMode: Main.KeybindingMode.SYSTEM_MODAL,
+                                        keybindingMode: Shell.KeyBindingMode.SYSTEM_MODAL,
                                         shouldFadeIn: true });
 
         this.state = State.CLOSED;