Fix transient_for endless loop

Other approaches would be:
- Slow/fast pointer technique.
- Using a set/associative map to save 'seen' nodes. i3 does not have
  such data structure.

Counting the total amount of windows is the simpler to implement.

I've also extracted the logic in a function and re-used it in render.c.

Fixes #4404

include/con.h

@@ -208,6 +208,14 @@ Con *con_by_frame_id(xcb_window_t frame);
  */
 Con *con_by_mark(const char *mark);
 
+/**
+ * Start from a container and traverse the transient_for linked list. Returns
+ * true if target window is found in the list. Protects againsts potential
+ * cycles.
+ *
+ */
+bool con_find_transient_for_window(Con *start, xcb_window_t target);
+
 /**
  * Returns true if and only if the given containers holds the mark.
  *