KCM KWinTabbox disable widget if property is immutable.

Summary:
Some widget cannot be managed by KCModule, so we have to disable the widget if the property is immutable.
Remove unused parameter in `createConnections()`

Test Plan: Add [$i] right after the key or group in kwinrc, the widget should be disabled.

Reviewers: #kwin, ervin, bport, meven, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28484