Makes custom JS objects compatible with SpiderMonkey ESR31.

In v24 you called JS_InitClass and passed in a definition of JSNative
functions. Later you could call JS_NewObject with this class and the
object would get a prototype with the specified JSNative functions.
In ESR31 you now have to explicitly store the prototype object returned
by JS_InitClass and pass it as prototype argument to JS_NewObject to
achieve the same.
This change modifies our existing ScriptInterface implementation for
custom object types a bit and uses it at places where the JSAPI was used
directly before.

Refs #2462

This was SVN commit r15524.

source/scriptinterface/ScriptInterface.h

@@ -413,8 +413,8 @@ private:
 	class CustomType
 	{
 	public:
-		JSObject *	m_Object;
-		JSClass *	m_Class;
+		JSObject*	m_Prototype;
+		JSClass*	m_Class;
 		JSNative 	m_Constructor;
 	};
 	void Register(const char* name, JSNative fptr, size_t nargs);