Add support for interactive selection through touch events

Summary:
This was forgotten during implementing the interactive window/point
selection. With this change it is also possible to perform the
interaction through touch events. In that case KWin takes over any
existing touch sequence.

This implements T5315.

Test Plan: Added auto test and took a screenshot through touch events

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5203