QUrl::toLocalFile doesn't handle qrc:// URLs, do like
QQuickImageBase instead and use a public utility function
to does that conversion.

This allows loading qtwebchannel.js and use it in subsequent
user scripts.

Change-Id: I4791abdc96a7cd1e65ff76b1adeab9f9b250b582
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>