Handle keyboard in action selector OSD

Summary:
The OSD is not all that bad, but it was only usable with the mouse.
Change it so that left-right and tab navigates and space/return confirms
the selection.

BUG: 395804

Reviewers: #plasma, davidedmundson

Subscribers: broulik, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14165