Add "validate_property" virtual func

- Change back privacy of StringName::from_opaque method.

Author: Yarwin

godot-core/src/builtin/string/string_name.rs

@@ -56,7 +56,7 @@ pub struct StringName {
 }
 
 impl StringName {
-    pub(crate) fn from_opaque(opaque: sys::types::OpaqueStringName) -> Self {
+    fn from_opaque(opaque: sys::types::OpaqueStringName) -> Self {
         Self { opaque }
     }