refactor code for removing children from a con

Let’s see how this callback stuff will work out. If it doesn’t work out well,
we will remove it.

include/data.h

@@ -358,6 +358,9 @@ struct Con {
     TAILQ_ENTRY(Con) focused;
     TAILQ_ENTRY(Con) all_cons;
     TAILQ_ENTRY(Con) floating_windows;
+
+    /** callbacks */
+    void(*on_remove_child)(Con *);
 };
 
 #endif