Add a PlatformCursorImage to Platform and EffectsHandler

Summary:
There are several effects (screenshot, zoom) which need access to the
cursor image and cursor hotspot. So far these effects used X11
unconditionally to get the cursor which obviously does not work on
Wayland.

This change adds a new class PlatformCursorImage to kwinglobals which
wraps what a cursor is (image and hotspot) and adds a new virtual method
to Platform to provide such a PlatformCursorImage. By default it's the
cursor image the Platform tracks. On X11/standalone platform this new
virtual method is overriden and provides a PlatformCursorImage from X11
using the code previously used in screenshot effect.

Screenshot effect and zoom are adjusted to use the new API instead of
X11.

Test Plan:
Zoom effect tested on Wayland, now gets the proper cursor icon.
X11 functionality not yet tested.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3093