js: Refactor gjs_object_*_property()

This code was due for some refactoring, and changing GjsConstString to be
rooted in the following commit was the opportunity for it. We now have
gjs_object_get_property(), gjs_object_has_property(),
gjs_object_set_property(), and gjs_object_define_property() as wrappers
for JS_GetPropertyById(), etc., that take a GjsConstString constant
instead of a jsid.

In addition, we rename gjs_object_require_property_value() to be an
overload of gjs_object_require_property(); the old name was confusing
because the name _without_ "value" was the one that dealt with a
JS::Value!

Same rename for gjs_object_require_converted_property_value().

This whole thing allows us to get rid of some code, moving a bunch of
roots into these new functions. These roots are strictly speaking still
necessary, but in the next commit we're going to root all the interned
strings permanently, which should have been done all along.

https://bugzilla.gnome.org/show_bug.cgi?id=776966