[effects] Show info message in interactive window screenshot mode

Summary:
While taking an interactive screenshot we should inform the user about
what's going on, how to take the screenshot and how to cancel it.

For this the effect creates an EffectFrame with the required
information.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3403