Summary:
As is KWin only had 1 Cursor which was a singleton. This made it impossible for
us to properly implement the tablet (as in drawing tablets) support and show where
we're drawing.
This patch makes it possible to have different Cursors in KWin, it makes all the
current code still follow the mouse but the tablet can still render a cursor.

Test Plan: Tests pass, been using it and works as well as before but with beautiful tablet cursors.

Reviewers: #kwin, cblack, davidedmundson

Reviewed By: #kwin, cblack, davidedmundson

Subscribers: davidedmundson, cblack, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28155