use a property variable
define const strings for js commands

It allows to use referenced parameters in PostMessage methods.

Also rename : PostCmdSend() --> SendCmdToJs()

Signed-off-by: Corentin Lecouvey <corentin.lecouvey@open.eurogiciel.org>
comments

Signed-off-by: Corentin Lecouvey <corentin.lecouvey@open.eurogiciel.org>