Summary:
Use the new functionality in KWayland to support drag and drop via touch
screens.

Either a drag and drop session with pointer or touch is possible, but not
both at the same time. Pointer/touch gets deactivated if a touch/pointer
drag and drop session is active.

Test Plan: Manually.

Reviewers: #kwin, davidedmundson

Subscribers: davidedmundson, alexde, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15466