Support touch events in DecorationEventFilter

Summary:
Touch events are emulating mouse events, in particular left mouse
button.

With this change one can move windows through the decoration, use
the decoration buttons and also support the double click action.

As finding the decoration is pretty much exactly the same as for
pointer events, a new base class is introduces which provides the
functionality of updating the decoration and the shared common
variables.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1604