Send a pointer leave when triggering a move resize

Summary:
When triggering a move resize all following pointer events are grabbed
by KWin itself. Thus the correct behavior is to informe the client about
it and send a pointer leave.

This ensures that after the move resize ended the pointer gets a new
enter. By sending anew pointer enter the position gets updated to the
new position which so far did not happen and the client generated events
on the wrong position.

BUG: 371573
FIXED-IN: 5.8.3

Reviewers: #kwin, #plasma_on_wayland, broulik

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3154