Add support for on-screen-message to kwineffects

Summary:
This adds support for the new on-screen-message support to the
kwineffects API and makes use of it in the colorpicker effect.

Not yet added to screenshot effect as for that we need support for
skip close animation on the on-screen-message window.

Reviewers: #plasma, #kwin

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3803