Modify _tree_next() so that when we reach the workspace container:
1. Find the next corresponding output (screen) using the added
get_output_next().
2. If there is another output, find the visible workspace.
3. Call workspace_show on found workspace.
4. Find the appropriate window to focus (leftmost/rightmost, etc.) using
con_descend_direction, and then focus it.
I've only tested on horizontal monitors (left/right).