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