* API kept the same (Although functions could be renamed to set_metadata/get_metadata in a later PR), so not much should change.
* Metadata now exposed as individual properties.
* Properties are editable in inspector (unless metadata name begins with _) under the metadata/ namespace.
* Added the ability to Add/Remove metadata properties to the inspector.
This is a functionality that was requested very often, that makes metadata work a bit more similar to custom properties in Blender.
* Very old macros from the time Godot was created.
* Limited arguments to 5 (then later changed to 8) in many places.
* They were replaced by C++11 Variadic Templates.
* Renamed methods that take argument pointers to have a "p" suffix. This was used in some places and not in others, so made it standard.
* Also added a dereference check for Variant*. Helped catch a couple of bugs.
