Added 'con_is_hidden' to check whether a given container is visible to the user assuming its workspace is visible.

This is useful for determining whether we want to set the _NET_WM_STATE_HIDDEN atom on the window.

include/con.h

@@ -42,12 +42,19 @@ bool con_is_leaf(Con *con);
  */
 bool con_has_managed_window(Con *con);
 
-/*
+/**
  * Returns true if a container should be considered split.
  *
  */
 bool con_is_split(Con *con);
 
+/**
+ * This will only return true for containers which have some parent with
+ * a tabbed / stacked parent of which they are not the currently focused child.
+ *
+ */
+bool con_is_hidden(Con *con);
+
 /**
  * Returns true if this node has regular or floating children.
  *