* Exposed functions in Engine to register and unregister singletons.
* Added the concept of user singletons, which can be removed (the system ones can't).
Adds ability to add error return documetation to the binder and class reference.
Usage example:
```C++
void MyClass::_bind_method() {
[..]
BIND_METHOD_ERR_RETURN_DOC("load", ERR_FILE_CANT_OPEN, ERR_FILE_UNRECOGNIZED);
}
```
One function of ConfigFile was changed as example.
* Allows calling into native extensions directly with a pointer
* Makes it easier to implement some APIs more efficiently
* Appears with a "*" in the documentation for the argument.
* Implementing the pointer handling is entirely up to the implementation, although the extension API provides some hint.
* AudioStream has been implemented as an example, allowing to create NativeExtension based AudioStreams.
* `_gui_input`, `_input`, `_unhandled_input` and `_unhandled_key_input` are now regular C++ virutal functions.
* Everything else converted to GDVIRTUAL
* BIND_VMETHOD is gone, always use the new syntax from now on.
Creating `_gui_input` method and using the binder to register events will no longer work, simply override the virtual function now.
This commit adds quite a chunk of modifications to particles
- particle (value + randomness) now use min and max instead
- passing a curveXYZtexture is now possible and will scale particles per-axis
- CPUParticle3D have an optional parameter to split the scale curve per-axis
* New syntax is type safe.
* New syntax allows for type safe virtuals in native extensions.
* New syntax permits extremely fast calling.
Note: Everything was replaced where possible except for `_gui_input` `_input` and `_unhandled_input`.
These will require API rework on a separate PR as they work different than the rest of the functions.
Added a new method flag METHOD_FLAG_OBJECT_CORE, used internally. Allows to not dump the core virtuals like `_notification` to the json API, since each language will implement those as it is best fits.
Remove duplicate bezier code and use Curve instead.
Add an overridable method for retrieving the points of a connection line, which
makes it posible to create custom connections lines.
* Extensions are now scanned and loaded on demand.
* Extensions found are cached into a file that is used to load them (which is also exported).
* Editor will ask to restart when an extension requires core functionality.
* Editor will attempt to load extensions always before importing or loading scenes. This ensures extensions can register the relevant types.
