Facilitate exposing platform-exclusive interfaces to all platforms

This makes the interfaces available, without implementation, in other
platforms and the editor, which facilitates documenting platform-exclusive
classes.

Platform-exclusive APIs must be set up in platform/<platform>/api/api.cpp.
Provide noop method-implementations where necessary.

Also setup and document the HTML5 platform's JavaScript singleton.

platform/javascript/os_javascript.h

@@ -32,7 +32,6 @@
 
 #include "audio_driver_javascript.h"
 #include "drivers/unix/os_unix.h"
-#include "javascript_eval.h"
 #include "main/input_default.h"
 #include "os/input.h"
 #include "os/main_loop.h"
@@ -67,10 +66,6 @@ class OS_JavaScript : public OS_Unix {
 
 	PowerJavascript *power_manager;
 
-#ifdef JAVASCRIPT_EVAL_ENABLED
-	JavaScript *javascript_eval;
-#endif
-
 	static void _close_notification_funcs(const String &p_file, int p_flags);
 
 	void process_joypads();