With this change the time is correctly updated also when using
the VLC backend.
It worked for some reason with phonon-gstreamer, but it looks like
it's just by accident. In fact, according the documentation, the default
value of the property is 0:
https://api.kde.org/phonon/html/classPhonon_1_1MediaObject.html#ae0f0ab91c97a1f3495ed9467048bb81a
The choosen value (100) should allow for a fluid scrolling of the
progress element, without having a too small resolution, and
it seems to match the resolution of the gstreamer backend anyway.
BUG: 389907