Fix broken Qt Multimedia backend

Reason for the change

Fixes two bugs caused by changes in QtMultimedia 6

• Playing the same track twice in a row causes the playback to skip to the end of the track (it should play from the beginning)
• The media player stops when playing a track through to the end (it should start playing the next track in the playlist)

Test plan

• Add an album to the playlist
• Add another track to the end of the playlist twice

Follow the checklist (for VLC and Qt backends):

Repeat One (play any track)

• Play until the end -> track should repeat
• Press skip to next track -> next track should start playing
• Press skip to previous track -> previous track should start playing
• Press pause, press skip to next track -> next track should load but be paused
• Press pause, press skip to previous track -> previous track should load but be paused

Repeat All (play any track)

• Play until the end -> next track should start playing
• Press skip to next track -> next track should start playing
• Press skip to previous track -> previous track should start playing
• Press pause, press skip to next track -> next track should load but be paused
• Press pause, press skip to previous track -> previous track should load but be paused

Repeat None (play first track)

• Play until the end -> next track should start playing
• Press skip to next track -> next track should start playing
• Press skip to previous track -> nothing happens
• Press pause, press skip to next track -> next track should load but be paused
• Press pause, press skip to previous track -> nothing happens

Repeat None (play last track)

• Play until the end -> first track loads, playback stops
• Press skip to next track -> nothing happens
• Press skip to previous track -> previous track should start playing
• Press pause, press pause, skip to next track -> nothing happens
• Press pause, press skip to previous track -> previous track should load but be paused

Repeat None (play any other track)

• Play until the end -> next track should start playing
• Press skip to next track -> next track should start playing
• Press skip to previous track -> previous track should start playing
• Press pause, press skip to next track -> next track should load but be paused
• Press pause, press skip to previous track -> previous track should load but be paused

Repeat None (play first of same track queued twice)

• Play until the end -> next track should start playing
• Press skip to next track -> next track should start playing
• Press skip to previous track -> previous track should start playing
• Press pause, press skip to next track -> next track should load but be paused
• Press pause, press skip to previous track -> previous track should load but be paused

Bugs fixed

BUG: 479130

BUG: 480743