Piano: Add velocity and pitch support

As Piano will later move to the RollNote defintions of LibDSP, it's a
good idea to already insert velocity and pitch support, even though it's
currently not used.

Userland/Applications/Piano/Music.h

@@ -42,6 +42,8 @@ struct RollNote {
 
     u32 on_sample;
     u32 off_sample;
+    u8 pitch;
+    i8 velocity;
 };
 
 enum Direction {