add some metadata

Summary: accessible by the qml implementations

Test Plan:
the properties kcm.name and kcm.description are accessible from the qml part of
the cursortheme kcm

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D8691