This patch correctly determines the previously focused workspace on the
target output when moving a workspace to another output. Before, we used
nodes_head for this, which will not actually return the previously focused
workspace, but just the first workspace on that output. Hence, we now use
focus_head instead.
This bug was introduced all the way back in 1e143fea when the feature of
moving workspaces to another output was first implemented.
fixes#2229
