Use GString: call_js_listeners, call_js_callback
Use GString: call_js_listeners, call_js_callback

With support for generating arbitrarily large monitor data strings in
the last commit, we need to actually be able to call the JS listeners
and callbacks with that.  So here's the same for the buffers of that.

Good for the future too

anko/hudkit,anko/hudkit,anko/hudkit