[effects/translucency] Cancel existing animations before starting new

Summary:
It can happen that startAnimation is invoked multiple times for a
window. In case it was invoked a second time the previous animation was
not cancelled. This resulted in the set-animation to never end. When
closing a window, it would stay around as a translucent, non-interactive
window zombie.

This change ensures that existing animations get cancelled.

BUG: 342716
FIXED-IN: 5.8.3

Test Plan: Tested through autotest and manually.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3190