Send done event after the number of rows has been changed

Summary:
The plasma virtual desktop protocol states the following about
the done event

> This event is sent after all other properties has been sent after
> binding to the desktop manager object and after any other property
> changes done after that.

Thus we have to send that event when the number of rows has been changed.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18518