Don't crash on printing nested types

When adding an Array or Dictionary to itself operator String() got in an
infinite loop. This commit adds a stack to operator String() (Through
the use of a new 'stringify method'). This stack keeps track of all
unique Arrays and Dictionaries it has seen. When a duplicate is found
only a static string is printed '[...]' or '{...}'.

This mirror Python's behavior in a similar case.

core/dictionary.cpp

@@ -270,6 +270,10 @@ void Dictionary::operator=(const Dictionary &p_dictionary) {
 	_ref(p_dictionary);
 }
 
+const void *Dictionary::id() const {
+	return _p->variant_map.id();
+}
+
 Dictionary::Dictionary(const Dictionary &p_from) {
 	_p = NULL;
 	_ref(p_from);