SpiderMonkey-related changes in preparation for the upgrade to SpiderMonkey 45, refs #4893.

- Remove JSVAL_ZERO and JSVAL_NULL
https://bugzilla.mozilla.org/show_bug.cgi?id=1177825
- Remove *_TO_JSVAL https://bugzilla.mozilla.org/show_bug.cgi?id=1177892
- Drop support for parent object in the deserializer
https://bugzilla.mozilla.org/show_bug.cgi?id=1136345,
https://bugzilla.mozilla.org/show_bug.cgi?id=805052
- Correctly use boolean values in JS::RuntimeOptionsRef
- Use JS_FN instead of JS_FS: in future versions, JS_FS is not public
because it isn't supposed to be used in JSAPI code
- Allow to select flags for global objects, and correctly mark progress
bars in the loading screen as not readonly+permanent
- Remove empty JSI_props in IGUIObject

Reviewed By: wraitii, elexis
Differential Revision: https://code.wildfiregames.com/D1716
This was SVN commit r22052.

source/ps/scripting/JSInterface_VFS.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -99,7 +99,7 @@ JS::Value JSI_VFS::BuildDirEntList(ScriptInterface::CxPrivate* pCxPrivate, const
 	BuildDirEntListState state(cx);
 	vfs::ForEachFile(g_VFS, path, BuildDirEntListCB, (uintptr_t)&state, filter, flags);
 
-	return OBJECT_TO_JSVAL(state.filename_array);
+	return JS::ObjectValue(*state.filename_array);
 }
 
 // Return true iff the file exits
@@ -157,7 +157,7 @@ JS::Value JSI_VFS::ReadFileLines(ScriptInterface::CxPrivate* pCxPrivate, const s
 
 	CVFSFile file;
 	if (file.Load(g_VFS, filename) != PSRETURN_OK)
-		return JSVAL_NULL;
+		return JS::NullValue();
 
 	CStr contents = file.DecodeUTF8(); // assume it's UTF-8