Correctly count the number of windows for no_focus. (#2296)

Previously we counted the number of (direct) children of the workspace to
decide whether no_focus should be applied or not. However, this doesn't
work correctly if there's a single container with multiple windows on the
workspace.

This patch correctly counts all windows on the workspace.

fixes #2292

include/con.h

@@ -200,6 +200,12 @@ Con *con_for_window(Con *con, i3Window *window, Match **store_match);
  */
 int con_num_children(Con *con);
 
+/**
+ * Count the number of windows (i.e., leaf containers).
+ *
+ */
+int con_num_windows(Con *con);
+
 /**
  * Attaches the given container to the given parent. This happens when moving
  * a container or when inserting a new container at a specific place in the