Implement switching focus across screens.

Implement switching focus across screens.

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).