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.

docs/userguide

@@ -2053,6 +2053,12 @@ parent::
 child::
 	The opposite of +focus parent+, sets the focus to the last focused
 	child container.
+next|prev::
+	Automatically sets focus to the adjacent container. If +sibling+ is
+	specified, the command will focus the exact sibling container,
+	including non-leaf containers like split containers. Otherwise, it is
+	an automatic version of +focus left|right|up|down+ in the orientation
+	of the parent container.
 floating::
 	Sets focus to the last focused floating container.
 tiling::
@@ -2068,6 +2074,7 @@ output::
 <criteria> focus
 focus left|right|down|up
 focus parent|child|floating|tiling|mode_toggle
+focus next|prev [sibling]
 focus output left|right|up|down|primary|<output>
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