Make the action selector OSD independent of the other OSDs

Summary:
This OSD should handle keyboard input and behave different, so make it work on its own.
In follow up changes, the keyboard handling will be added.

CCBUG: 395804

Reviewers: #plasma, davidedmundson

Subscribers: broulik, davidedmundson, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14143