[libkwineffects] Port AnimationEffect to new connect syntax
Summary: Connecting with the new syntax is faster and also type checked.
Test Plan: Scripted effects still work as expected.
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18393