Unify focus abstractions for internal and wayland windows

This simplifies focus related logic a bit. Instead of differentiating between wayland and internal window focus, simply maintain window focus that works both with regular wayland windows as well as the ones created by kwin.

It would be nice to abstract event dispatching for windows and decorations, not sure if it's possible, e.g. add QObject *InputDeviceHandler::focusObject() to represent a decoration or a surface and have the input device handler forward events to it.

cc #67