save default cover position

The reason why jus is seeing a hugh spike in CPU load is because we constantly look for the track cover even if we can't find it.  We never save that we already have seen the track and marked it with 'defaultCover'.

This patch already partially fixes it. On the first run the coverdelegate is showing the default-cover for tracks where we couldn't find a cover. This does not happen on the second run. This is because we don't do a database save until we shutdown mixxx or scanned 500 tracks. My sample library has ~30 tracks.

My current quickfix for that is to check if the images are equal. But that means converting the pixmaps everytime and doing what ever algorithms Qt uses to check of the images are equal.
but it seems that it'll never be triggered - see line 68
+ do you wanna show the default cover?
+ image/pixmap conversion is a slow process - we should not place it in this paint() method