Prevent Amarok from stalling if the standard playing mode is used and previous file does not exist

This issue was talked about in (bug 438651) Amarok stalls if the Previous Track button is pressed but the file not longer exists

TEST PLAN:

- Start Amarok, press the Track Progression button and enable Standard. Check that no other option there is enabled.
- Copy some track files to /tmp.
- Add a track from /tmp to the Amarok playlist, start playing and check that there is no way of going to a previous or next track.
- Add another track to the Amarok playlist, start playing and check that there is a way of going to the previous or to the next track, but not both.
- Add more tracks to the Amarok playlist, play the second track, check that there is a way of going to the previous and to the next track.
- Perform several other tests that you consider that are necessary.
- Delete a track file in /tmp, check that in Amarok the related track is seen using another color, and that the previous and next track buttons work without Amarok getting stopped. Go deleting more tracks in /tmp and repeating the test until there are no more tracks files left to be deleted in /tmp, checking that after each step Amarok does not stall.                    
- Press the Track Progression button, enable the Repeat List option and perform similar tests.