Don't advertise on Wayland features that aren't available

Summary:
It's quite stressful and it clearly doesn't push developers to implement the features.

CCBUG: 386271
CCBUG: 377157

Test Plan: Now I don't choose to take a screenshot of the active window and get sad when I can't.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: hein, ngraham, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D17082