Hide mouse cursor when interacting with touch screen

Summary:
On touch events the mouse cursor gets hidden, on next mouse event the
mouse cursor is shown again. This IMHO significantly improves the system
interaction if touch is the primary user interface.

Test Plan: Test case added and ctest passes

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17280