* 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.
* Deprecates GDNative in favor of a simpler, lower level interface.
* New extension system allows registering core engine classes.
* Simple header interface in gdnative_interace.h
