Add way to hide/show cursor in Platform

Summary:
Some platforms support to hide and show the cursor. This will be needed
by e.g. the zoom effect which currently only provides this functionality
on X11.

This change introduces a new method in the Platform to hide and show the
cursor. The methods need to be called balanced and the implementation
takes care of only showing again if all hide got matched by a show.

The actual hiding and showing is performed in the platform plugins. So
far the DRM and X11/Standalone platforms implement the required
functionality, though other platforms probably could implement as well.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3119