Replace deprecated KConfigDialogManager::changedMap with property in the class definition

Summary:
KConfigDialogManager::changedMap is deprecated since KF 5.32. This commit mimics the replacement commit from Okteta:

https://cgit.kde.org/okteta.git/commit/?id=1ed966965b656bac22e1fe99e1b7ede00da68263

Test Plan: Compiles. It seems that Equation editor behavior does not change.

Reviewers: #kde_edu, aacid

Reviewed By: aacid

Subscribers: kossebau, aacid, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D25078