Make QQmlAbstractUrlInterceptor public

A supporting class for file selectors and qml import control, it allows
redirecting local file paths (use a QNAM for remote paths).

All work was done previously for Qt 5.1, including tests. This is just
making it public for Qt 5.2.

Change-Id: Icb319cc23200541c8e4088b2d400e9af94f3eee5
Reviewed-by: Lars Knoll <lars.knoll@digia.com>