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).
Due to lots of cases which were added and added to tree_move(), the function
was not really easy to understand. For this refactoring, I wrote tree_move()
from scratch, thinking about (hopefully) all cases. The testsuite still passes.
The move command also has different parameters now. Instead of the hard to
understand 'before v' stuff, we use 'move [left|right|up|down]'.
Quote from the source:
When the container type is CT_WORKSPACE, the user wants to change the
whole workspace into stacked/tabbed mode. To do this and still allow
intuitive operations (like level-up and then opening a new window), we
need to create a new split container. */
