Add missing property to KKeySequenceWidget

Summary:
Oddly enough we had all the necessary to manage keySequence as a
property but that was never exposed. This makes it for instance
unavailable to Qt Designer and impossible to use with KConfigXT.

Reviewers: Frameworks, dfaure, davidedmundson, bport

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25208