Force reevaluating state on pending deletion changes

Summary:
This was missing and thus the apply button wouldn't get enabled in case
of a pending deletion. This is due to this mechanism being outside of
the KConfigXT realm.

Reviewers: #plasma, bport, mart, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25076