workspacesView: initialize self._restackedNotifyId

This attribute was previously only assigned in show(). hide() compares
this attribute to 0. If hide() is called before show() is first called,
the comparison would give the correct result (undefined > 0 is false)
but log a warning:

    JS WARNING: [resource:///org/gnome/shell/ui/workspacesView.js 529]:
    reference to undefined property "_restackedNotifyId"

Initialize this attribute in _init(), alongside _scrollEventId and
_keyPressEventId which are also used in hide().

(cherry picked from commit b2fabd9356)

js/ui/workspacesView.js

@@ -470,6 +470,7 @@ var WorkspacesDisplay = new Lang.Class({
         this._switchWorkspaceNotifyId = 0;
 
         this._notifyOpacityId = 0;
+        this._restackedNotifyId = 0;
         this._scrollEventId = 0;
         this._keyPressEventId = 0;