Fix keyboard focus issues when setting track rating

ListBrowserDelegate & PlayListEntry: Set RatingStar.readOnly correctly

Previously this was hard-coded to false, so non-editable rating stars would grab keyboard focus. This meant that the user had to tab through each rating star just to leave the list browser/playlist.

ListBrowserDelegate & PlayListEntry: stop editing the track rating when focus is lost

So if you start rating and then click a button or navigate away from the track via keyboard, the track is no longer in the "rating is being edited" status