SoundBuffer: Set song volume

- Requires a new column in the database after preview: `volume` REAL
- The value is a volume multiplier, if the value is set to null or 1 there will be no change
- The volume can be set in debugger
- Imported TJA files are now read from disk every time the song is played, freeing some memory and making it easier to create charts
- Correctly parse TJA files with alphabet notes, added "A" and "B" notes, which appear as DON (Big) and KA (Big) respectively

public/src/js/scoresheet.js

@@ -856,7 +856,7 @@ class Scoresheet{
 		this.draw.clean()
 		this.canvasCache.clean()
 		assets.sounds["bgm_result"].stop()
-		snd.musicGain.fadeIn()
+		snd.buffer.loadSettings()
 		this.redrawRunning = false
 		pageEvents.keyRemove(this, "all")
 		pageEvents.remove(this.canvas, ["mousedown", "touchstart"])