Make it possible to set a web-channel so that it can only be accessed
by private scripts.

Pulls in needed API extension in 3rdparty.

Task-number: QTBUG-50318
Change-Id: I61bcce5c318dffe0a406ee8cddf31f58a021c22c
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>