Draft: effects/overview: Allow forwarding pointer input to panel

One thing that is sort of expected to work in the overview effect is working input in the panel. With this change, kwin would forward mouse events to the panel so user can launch apps, etc.

This feature is available only on Wayland. Input redirection on X11 is a very difficult problem as kwin is not in charge of input handling.

It's a quick and dirty patch to get things going. cc #67