* 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.
This commit adds a condition to VariantCaster that casts Variants of type OBJECT to any type T, if T is derived from Object.
This change enables a fair bit of code cleanup. First, the Variant implicit cast operators for Node and Control can be removed, which allows for some invalid includes to be removed. Second, helper methods in Tree whose sole purpose was to cast arguments to TreeItem * are no longer necessary.
A few small changes also had to be made to other files, due to the changes cascading down all the includes.
