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