[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