Wayland: when blocking remember latest config change instead of crashing

Summary:
Configuration change requests by clients while the Wayland server is
processing a change lead to a failing assert in blockSignals().

Instead in such a situation remember the latest configuration change request
and apply it after the current change has been processed.

Test Plan:
Auto test exposing the problem. Also manually with my output color correction
code.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12517