Support for window activation tokens

[nicolasfella]

Quite often you want to raise an existing window of the first instance when attempting to start a second one.

On X11/Wayland one needs to pass a startupId/activation token from the new to the existing instance. This is doable in KDSingleApplication via sendMessage.

However that mean the consumer has to deal with all the window system specific code themselves. KDSingleApplication could provide helpers to deal with that and abstract such platform details