Prompt user to save/discard changes upon closing config dialog

Summary:
When the configuration dialog is closed with unsaved changes,
a message box is prompted to save/discard them or cancel the event.

BUG: 391206

Reviewers: Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: ngraham, elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D19904