Introduce C++ templates replacements for DEFINE_INTERFACE_X and RegisterFunction macros

The new methods:
- aren't included in ScriptInterface.h directly, lightening that header
- don't use boost CPP
- don't need argument types or number or constness to be specified
- can work with object methods somewhat transparently
- support optional cmptPrivate (allowing removal of many UNUSED macro)
- support optional const ScriptRequest&, which is safer.

This first diff changes only some of the JSI files & the component
manager. Further diffs will update other files and finally delete the
current code.

Differential Revision: https://code.wildfiregames.com/D2818
This was SVN commit r24969.

source/scriptinterface/ScriptInterface.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2020 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -83,6 +83,7 @@ public:
 	JS::Value globalValue() const;
 	JSContext* cx;
 	JSObject* glob;
+	JS::HandleObject nativeScope;
 private:
 	JS::Realm* m_formerRealm;
 };