Support touch events on KWin internal windows

Summary:
Qt's touch event API is rather difficult and complex to implement.
As none of KWin's internal windows supports multi-touch gestures yet,
this is going the easy route and just simulates a left mouse button
press. If in future need arises for touch gesture support on KWin's
internal windows, this can be added.

Test Plan: Tested on exopc with DebugConsole and auto test

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1661