Support for assets and qrc in QtMultimedia

Add support for reading media files from resource system.

The most important feature in this case would be to be able to load from qrc, as this is the preferred way of deploying resources in a Qt app. We would probably have to extract the file into a cache somehow and pass that to Android, but it should at least be transparent to the user. Right now, you get nothing when e.g. using relative paths inside a QML file, and there's no easy way of debugging this.