[kwineffects] Add hide/show cursor to EffectsHandler

Summary:
The implementation delegates to the Platform to perform the actual
show/hide of the cursor image.

This replaces the implementation in the zoom effect which so far
directly interacted with xfixes to show/hide the cursor. This is now
provided by the x11/standalone platform. And due to this change the zoom
effect can now properly hide the cursor on platform DRM (wayland) as
well.

Test Plan: Zoom effect on Wayland hides the cursor

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3120