Show config selection OSD when display button is pressed

Summary:
This makes the working of the display button much more intuitive.

FEATURE: 390096

Test Plan: Tested with external display plugged in, system behaves as expected

Reviewers: Plasma, dvratil

Reviewed By: dvratil

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10896