js: Initialize some properties

Otherwise those can result in the (harmless) "reference to undefined
property" warnings.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/970

js/ui/shellEntry.js

@@ -166,6 +166,7 @@ class CapsLockWarning extends St.Label {
 
         let seat = Clutter.get_default_backend().get_default_seat();
         this._keymap = seat.get_keymap();
+        this._stateChangedId = 0;
 
         this.connect('notify::mapped', () => {
             if (this.is_mapped()) {