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/tree.h

@@ -59,11 +59,16 @@ bool level_down(void);
 void tree_render(void);
 
 /**
- * Changes focus in the given way (next/previous) and given orientation
- * (horizontal/vertical).
+ * Changes focus in the given direction
  *
  */
-void tree_next(char way, orientation_t orientation);
+void tree_next(Con *con, direction_t direction);
+
+/**
+ * Get the previous / next sibling
+ *
+ */
+Con *get_tree_next_sibling(Con *con, position_t direction);
 
 /**
  * Closes the given container including all children.