Add support for windows being in multiple tags

Add support for windows being in multiple tags

wm_t -> tag_t -> dpy_t -> col_t -> row_t -> win_t -> win_wm_t

Add separate rows as the lowest level wm container:
  - Each row points to a single window
  - Each window may be pointed to by multiple rows, as long as the rows
    are in separate tags.

Add search functions for windows:
  - Windows can no longer have direct parents because they can be in
    multiple tags.
  - Use search function to find a window from the current tag, then
    return it's path (the list of parent containers).
  - This is slower, if it becomes a problem, the windows parents can be
    cached either in the win_wm_t structure, or parent pointers can be
    added to each of the containers.
  - Using search functions also alleviates some headaches involving
    updating the parent pointers.