scripting: Provide methods to create and remove virtual desktops

Summary: There are methods to create and remove virtual desktops via dbus, but I think it would be convenient to provide these methods for KWin scripts as well.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29786