[3.0] qt: add support for building with Qt6

The code still works with Qt 5.5, the minimum version we support. We don't have a maximum Qt version, so theoretically it's supposed to build with the latest Qt.

The configure script is not adapted to handle the case of Qt6 being built without pkg-config, as we do in VLC 4.0. But that could be useful for Linux distros if they want to build VLC 3 with Qt6.