[libkwineffects] Construct correct mouse event when forwarding to QtQuickView

Summary:
Original code (copied from Aurorae) created a QMouseEvent for
QHoverEvents. Whilst it apparently worked, it's technically wrong.

It's safer to have an explicit type check given we come from a downcast
QEvent* and then build the correct event.

Test Plan: Hovered some icons

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25207