[server] Touch drag support

Summary: Adds functionality to do drag and drop with touch screens.

Test Plan: Manually. Autotest planned.

Reviewers: #kwin, #frameworks, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15464