When specifying source property of a Presentation element in QML, support relative paths properly

Check if giving relative path to presentation in Presentation element source property works correctly. The expected behavior is that it works like Image element source property (https://doc-snapshots.qt.io/qt5-dev/qml-qtquick-image.html#source-prop), i.e. if the path is relative, it'll be relative to the .qml file of the owning component. It likely doesn't work like this currently. If it doesn't, fix it. Test with both plain file system and qrc.