inform outputconfiguration clients that a change has been applied

Summary:
After changing the output configuration, the client expects that it is informed
whether or not a new configuration has been applied (or failed). This was ommitted
so far, meaning that clients wouldn't know what happened in kwin.

Since we don't track if a setting failed yet, send the applied() signal regardless.

CCBUG:384733

Test Plan: Verified that the signal arrived in libkscreen after changing scale of an output

Reviewers: graesslin, davidedmundson

Reviewed By: davidedmundson

Subscribers: kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7910