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.

include/commands.h

@@ -182,6 +182,12 @@ void cmd_exec(I3_CMD, const char *nosn, const char *command);
  */
 void cmd_focus_direction(I3_CMD, const char *direction);
 
+/**
+ * Implementation of 'focus next|prev sibling'
+ *
+ */
+void cmd_focus_sibling(I3_CMD, const char *direction);
+
 /**
  * Implementation of 'focus tiling|floating|mode_toggle'.
  *