Move XFixes cursor change tracking into the x11 standalone platform

Summary:
A dedicated X11EventFilter is added and created from the X11Cursor in
case we have XFixes. This means some more X11 specific code is now only
on X11.

Test Plan: Only compile tested.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7843