This adds GDNative as a separate class type.
It can be used to interface with native libraries by
using "native calls", which can be registered by modules
(and in future other GDNative libraries?).
It also reworks the currently called "GDNativeScript" into a
"NativeScript" that just makes use of the new GDNative instead
of it being the component that implements that functionality.
Implements open_in_external_editor for subclasses of ScriptLanguage.
Add option 'Debug with external editor' to debug menu to control the behavoir of script opened by editor.
