[effects/showpaint] Use a shortcut to toggle the effect

Summary:
The Show Paint effect is useful when debugging repaint regions issued by
effects. The only headache with it is necessity to enable/disable it.
Consider the following workflow:

* Do some change to an effect;
* Compile KWin (or the effect);
* Go to System Settings and enable the Show Paint effect;
* Test effect, check repaint regions, etc;
* Disable the Show Paint effect;
* Go to the step 1.

This workflow is really exhausting. Also, when testing repaints in a
nested compositor, things become quite messy.

Because purpose of this effect is to debug repaints (and because this
effect is not meant for daily usage), I think that's fine to change
how it's activated.

This patch improves the workflow by changing the way how this effect
gets activated. Instead of enabling/disabling it, one can just use a shortcut
to activate or deactivate the effect.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: broulik, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15703