[effects/morphingpopups] Start animation on windowFrameGeometryChanged

Summary:
EffectsHandlerImpl::windowFrameGeometryChanged is emitted when the frame
geometry of a window has _actually_ changed.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26865