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/Clip.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Forward.h>
 #include <AK/RefCounted.h>
 #include <AK/SinglyLinkedList.h>
 #include <AK/Types.h>
@@ -50,6 +51,7 @@ public:
     {
     }
 
+    Optional<RollNote> note_at(u32 time, u8 pitch) const;
     void set_note(RollNote note);
     // May do nothing; that's fine.
     void remove_note(RollNote note);