[libkwineffects] Emit a signal when active fullscreen effect changed

Summary:
Behaviour of some effects depends on presence of active fullscreen effect.
For example, Dim Inactive effect brightens windows if there is an active
fullscreen effect. If active fullscreen effect has been changed, these effects
might need to do some setup work, e.g. schedule repainting, toggle direction
of a timeline, etc.

For what it's worth, because the Dim Inactive effect doesn't schedule
repainting after leaving Desktop Grid, windows aren't dimmed back. One
need to move mouse to trigger dimming.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13701