GDExtension: Fix _get_property_list not working correctly in parent classes

[Zylann]

Godot-side fix for godotengine/godot-cpp#1183

The issue got really long as I spent all the evening investigating it, so in summary:

Godot used to get properties from extensions in two "passes" (first code by Reduz, later code by different contributors, but latter one was not a complete fix). First, it got all properties declared in _bind_methods, for every level of inheritance hierarchy.
And only after that, it called the extension instance _get_property_list, which only returned procedural properties of the most derived class.

Two problems:

• If parent classes declare _get_property_list too, their properties didn't appear anymore, because the code only queried the most derived one;
• Since it came after ClassDB::get_property_list of every inheritance level, procedural properties ended up in the wrong class group in the inspector (always the most-base extension class).

This fix replicates the behavior of _get_property_listv by properly interleaving properties added with _get_property_list with those added in _bind_methods, so they all get returned, and in the right place.

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There will be a GodotCpp-side fix to apply as well, because it seems GodotCpp sometimes returns the parent class _get_property_list when it should not, causing procedural properties to still sometimes appear in the wrong group.

[dsnopek]

Thanks!

This looks good to me, and helps to fix the issue in my testing.

[YuriSizov]

Thanks!