inputMethod: Pass all key events through the current IM method

Even though we are using an "xkb" source, it still makes sense to
pass the event through the IBus simple engine, in order to let it
handle compose keys and ctrl+shift+[u|e].

https://gitlab.gnome.org/GNOME/gnome-shell/issues/115

Closes: #115

js/misc/inputMethod.js

@@ -188,8 +188,7 @@ var InputMethod = new Lang.Class({
     vfunc_filter_key_event(event) {
         if (!this._context || !this._enabled)
             return false;
-        if (!this._currentSource ||
-            this._currentSource.type == Keyboard.INPUT_SOURCE_TYPE_XKB)
+        if (!this._currentSource)
             return false;
 
         let state = event.get_state();