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/PeakControllerEffect/PeakControllerEffectControls.cpp

@@ -30,6 +30,9 @@
 #include "PeakControllerEffect.h"
 #include "Song.h"
 
+namespace lmms
+{
+
 
 PeakControllerEffectControls::
 PeakControllerEffectControls( PeakControllerEffect * _eff ) :
@@ -104,6 +107,4 @@ void PeakControllerEffectControls::saveSettings( QDomDocument & _doc,
 }
 
 
-
-
-
+} // namespace lmms