OSX: Don't show QtWebEngineProcess in the dock

Set the LSUIElement key as Chromium does in its helper app's
Info.plist to prevent seeing a jumping icon in the dock while the OS
waits in vain for QtWebEngineProcess to create a window.

Change-Id: I6c836621ec506fde04bc3825f64c49630a065351
Task-number: QTBUG-42955
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>