Refactor tree_next

- Makes `tree_next` not recursive.
- Adds `focus next|prev [sibling]` command. See (1.) and (2.) in
https://github.com/i3/i3/issues/2587#issuecomment-378505551 (Issue also
requests move command, not implemented here).
- Directional focus command now supports command criteria.

Wrapping is not implemented inside a floating container. This was also
true before the refactor so I am not changing it here.

parser-specs/commands.spec

@@ -146,6 +146,8 @@ state WORKSPACE_NUMBER:
 state FOCUS:
   direction = 'left', 'right', 'up', 'down'
       -> call cmd_focus_direction($direction)
+  direction = 'prev', 'next'
+      -> FOCUS_AUTO
   'output'
       -> FOCUS_OUTPUT
   window_mode = 'tiling', 'floating', 'mode_toggle'
@@ -155,6 +157,12 @@ state FOCUS:
   end
       -> call cmd_focus()
 
+state FOCUS_AUTO:
+  'sibling'
+      -> call cmd_focus_sibling($direction)
+  end
+      -> call cmd_focus_direction($direction)
+
 state FOCUS_OUTPUT:
   output = string
       -> call cmd_focus_output($output)