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>

plugins/DynamicsProcessor/DynamicsProcessorControls.cpp

@@ -33,6 +33,9 @@
 #include "Engine.h"
 #include "Song.h"
 
+namespace lmms
+{
+
 
 #define onedB 1.1220184543019633f
 
@@ -154,5 +157,4 @@ void DynProcControls::subOneClicked()
 }
 
 
-
-
+} // namespace lmms