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/SongManager.cpp

@@ -235,7 +235,7 @@ void SongManager::LoadStepManiaSongDir( RString sDir, LoadingWindow *ld )
 				ld->Paint();
 			}
 			Song* pNewSong = new Song;
-			if( !pNewSong->LoadFromSongDir( sSongDirName ) )
+			if( !pNewSong->LoadFromSongDir( sSongDirName, false ) ) // don't ignore cache
 			{
 				/* The song failed to load. */
 				delete pNewSong;
@@ -277,7 +277,7 @@ void SongManager::LoadGroupSymLinks(RString sDir, RString sGroupFolder)
 		RString	sSymDestination = msdF.GetParam(0,1);	// Should only be 1 vale&param...period.
 		
 		Song* pNewSong = new Song;
-		if( !pNewSong->LoadFromSongDir( sSymDestination ) )
+		if( !pNewSong->LoadFromSongDir( sSymDestination, false ) ) // don't ignore cache
 		{
 			delete pNewSong; // The song failed to load.
 		}
@@ -993,7 +993,7 @@ void SongManager::RevertFromDisk( Song *pSong, bool bAllowNotesLoss )
 	pSong->Reset();
 	const bool OldVal = PREFSMAN->m_bFastLoad;
 	PREFSMAN->m_bFastLoad.Set( false );
-	pSong->LoadFromSongDir( dir );	
+	pSong->LoadFromSongDir( dir, true ); // when reverting, ignore cache
 	/* XXX: reload edits? */
 	PREFSMAN->m_bFastLoad.Set( OldVal );