Workspace layout (rows) should be stored with workspace count and names

Currently the workspace layout (that is, the number of rows) is set by the pager panel plugin (using libwnck) which stores it in xfce4-panel's xfconf channel (in a way that is not discoverable, since the property name is dependent on the order the pager plugin was added to the panel).

The workspace count and workspace names are set in the workspace settings dialog, and are stored in xfwm4's xfconf channel, where xfwm4 sets them up.

This works on X11, even if it is confusing. This will not work on Wayland, as the compositor is the only thing that can change the workspace layout.

So my feeling is that the workspaces settings dialog should also set the workspace layout, and store it in xfwm4's xfconf channel, and xfwm4 should set _NET_DESKTOP_LAYOUT at the same time it sets _NET_DESKTOP_NAMES and _NET_NUMBER_OF_DESKTOPS.

Then this will work naturally (and less confusingly) on both X11 and Wayland, and it won't require that the user has a pager plugin in their panel for their desired arrangement of workspaces. (Note that it's not just the pager that cares about the layout; xfwm4's workspace navigation keyboard shortcuts do too).

For now I'm going to assume that we'll store the number of rows in xfconf at xfwm4:/general/workspace_nrows.