Fix several annoying edit mode bugs, mostly by fixing the Song copy

operator and the various uses of it.

- revert file from disk: keeps old BPM.  The BPMS on disk are added to
  the existing BPM information!
- edit mode turns X stepcharts into 2X when reverting
- sometimes edit mode loses an entire stepchart: open a stepchart,
  change to a new stepchart, exit without going back, original chart gone
- edit a chart, make a change in a different chart, exit from the first chart:
  change made in different chart doesn't revert
- normally, when you create a stepchart and don't save it after editting it,
  it is deleted.  however, if you create a stepchart, don't save it, and exit
  edit mode while looking at a different chart, the chart isn't deleted.

Some existing bugs are not fixed, though:
- delete the last stepchart for a step type.  CTD.
- Edit one of the fake doubles stepcharts for a song that doesn't have
  real doubles stepcharts.  Exit without doing anything.  CTD.  This is
  probably the same problem as the previous bug.
- delete a song or a steps that happens to be part of a trail.  Trail is now
  broken.  Some other edits should affect a trail as well, but don't.

stepmania/src/Trail.cpp

@@ -32,6 +32,11 @@ bool TrailEntry::operator== ( const TrailEntry &rhs ) const
 
 Steps *TrailEntry::GetSteps() const
 {
+	// TODO:  There are a few bugs with the way we do trails right now.
+	//   - If a steps (or an entire song) is deleted, possibly through the editor,
+	//     this section will fail.
+	//   - If the editor changes a stepchart's stats, that should also be reflected
+	//     in the trail.
 	return m_StepsID.ToSteps( pSong, false );
 }