Add rows info to the plasma virtual desktop protocol

Summary:
in order for the pager to work correctly and not having to use a weird mix of wayland and dbus apis, the rows
number needs to be in the protocol

Test Plan: new passing autotest

Reviewers: #plasma, #kwin, davidedmundson

Reviewed By: #plasma, #kwin, davidedmundson

Subscribers: zzag, davidedmundson, hein, kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D17691