Even if NETWM spec says that the Desktop Layout is owned and maintained by a pager and not by the window manager, I disagree. It has been causing problems for years and is just wrong. Therefore this patch:

* moves the configuration of the number of rows from Pager Config UI to the Virtual Desktops KCM
* Number of rows is stored in the desktop section of KWin config file
* Virtual Desktops KCM updates the layout on change
* KWin sets the desktop layout on startup (as it sets the number of desktops and names)
* Pager gets the number of rows from the NetRootInfo instead of its own config
* Pager connects to KWin's reconfigure dbus signal to update the layout (KWindowSystem does not emit a signal)

This renders a major break with the NetWM spec:
* There is no process claiming the pager management selection any more
* If another pager is used together with KWin, KWin will ignore the settings of the pager
* If another window manager is used together with Plasma, the layout used by the window manager may not reflect the layout shown in the pager

As I think the concept of having Pager setting the desktop layout is broken, I will inform the NetWM spec mailing list when this patch is merged into master and recommend a change in the spec. Given that both GNOME Shell and Unity do have a combined Window Manager/Desktop Shell combination I expect them to ignore this part of the spec anyway.

Unrelated question: what is the use case of pager in a non-X11 environment and why are there ifdefs?

* KWin loads with the last saved desktop layout
* Changes in KCM are reflected in KWin and pager

Not tested:
* multiple pagers
* vertical pagers