Merge setting up abstractclient and x11client connections in EffectsHandler

Summary: Toplevel::paddingChanged() is not an X11-specific signal.

Test Plan: Relevant tests still pass.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29520