Use KWin's D-Bus interface to get desktop layout rows on Wayland

Summary:
While I think mixing IPC systems in this class is not the way to go
long-term, we have a strong desire to ship a complete user experience
for virtual desktops on Wayland in 5.15. As D17691 has been delayed
and did not make the Frameworks 5 release we can depend on for 5.15,
this is Plan B.

Reviewers: Plasma, davidedmundson, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18290