Fix the Phonon notification backend not playing any sound

Phonon won't report its capabilities before a player has been created,
which leads to Quassel thinking that it can't play audio since we checked
for that before instantiating the player.

Some reordering in the ctor fixes this.
Avoid Phonon's deprecation warnings

MediaSource prefers a QUrl now. This shouldn't break with older Phonon
versions, I hope, since it seems MediaSource(QUrl) has been around since
forever.