Register KKeySequenceWidget to KConfigDialogManager

Summary:
This makes it usable in a KConfigXT context.

Depends on KKeySequenceWidget property to be available,
introduced with: https://phabricator.kde.org/D25208

Reviewers: Frameworks, dfaure, davidedmundson, bport

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25209