Allow booleanization of all types

We now allow booleanization of all types. This means that empty versions
of all types now evaluate to false. So a Vector2(0,0), Dictionary(),
etc.

This allows you to write GDScript like:
if not Dictionary():
  print("Empty dict")

Booleanization can now also no longer fail. There is no more valid flag,
this changes Variant and GDNative API.

core/variant.h

@@ -390,7 +390,7 @@ public:
 	uint32_t hash() const;
 
 	bool hash_compare(const Variant &p_variant) const;
-	bool booleanize(bool &valid) const;
+	bool booleanize() const;
 
 	void static_assign(const Variant &p_variant);
 	static void get_constructor_list(Variant::Type p_type, List<MethodInfo> *p_list);