Piano: Fix insertion and deletion of notes

On mouse move the pressed button is not present in the event argument
which causes the corresponding code to never fire. Instead it now stores
the original mouse down event and acts according to that on mouse move.

Userland/Libraries/LibDSP/Track.h

@@ -80,6 +80,7 @@ public:
     bool check_processor_chain_valid() const override;
     ReadonlySpan<NonnullRefPtr<NoteClip>> notes() const { return m_clips.span(); }
 
+    Optional<RollNote> note_at(u32 time, u8 pitch) const;
     void set_note(RollNote note);
     void remove_note(RollNote note);