The Effect class is extended by three new virtual methods:

• touchDown
• touchMotion
• touchUp

The methods return a boolean value so that the events can be filtered
out. E.g. an effect which has also a mouse grab installed wants to
filter out all events, other effects don't need the events exclusively.

This is a difference to how e.g. keyboard and pointer events are handled.
But is more close to how KWin's internal input event passing works and
makes it easier to get touch event: one does not explicitly has to grab
the events. It's also closer to Wayland where all input events are
available.

As a first example the Present Windows effect is adjusted and allows to
activate windows through the touch screen. As much code as possible is
shared with pointer input.