Namespace lmms (#6174)

This PR places all LMMS symbols into namespaces to eliminate any potential future name collisions between LMMS and third-party modules.

Also, this PR changes back `LmmsCore` to `Engine`, reverting c519921306 .

Co-authored-by: allejok96 <allejok96@gmail.com>

include/Note.h

@@ -33,6 +33,11 @@
 #include "SerializingObject.h"
 #include "TimePos.h"
 
+
+namespace lmms
+{
+
+
 class DetuningHelper;
 
 
@@ -247,4 +252,6 @@ private:
 typedef QVector<Note *> NoteVector;
 
 
+} // namespace lmms
+
 #endif