[effects] Port to new connect syntax

Summary:
The new connect syntax has several advantages over the old syntax:

(a) Connecting with the new syntax is faster;
(b) It is compile time checked.

There are still a few places where the old connect syntax is used, e.g.
connecting to QML buttons in the Desktop Grid effect.

Test Plan:
Have been testing this patch for ~2 weeks, haven't noticed any
regressions.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, broulik, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18368