Make it possible to override default source URL for Ministro

Ministro will by default download the XML files for how to find and verify libraries from a hardcoded URL in QtActivity.java (which is copied from Qt). This is currently a KDE url, but will be updated to be a qt-project url instead.

For commercial packages, it will be necessary to override this with another url. This can be done by changing the qt_sources array in libs.xml after copying it from Qt. I think it would be great if Qt Creator could be built with a compile-time option to automatically replace this with something else when the project is created/updated?