Fix sidebar view regression of not prompting about unsaved changes

Summary:
When you switch module with unsaved changes the other modes prompt if
you want to save the changes. Sidebar mode did not.

BUG: 392160

Test Plan:
Made a change, clicked another module. Got a prompt.

Clicked apply, it applied then changed

Tried again

Clicked cancel, it didn't change

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12686