actually draw drag icons in the mouse cursor during drags

Summary:
paint together the cursor image and the extra dnd image if available
on X11 with QDrag::setHotspot is possible to control their relative
position, which doesn't seem to have a wayland protocol correspondence
so their relative position are controlled just by the hotspot of the cursor itself

Test Plan:
folder graphics from dolphin is painted correctly

Reviewers: Plasma, KWin, davidedmundson

Reviewed By: Plasma, KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27174