Determine focused workspace correctly when moving workspace to output.

Determine focused workspace correctly when moving workspace to output.

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