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/DynamicsProcessor.cpp

@@ -32,6 +32,10 @@
 #include "embed.h"
 #include "plugin_export.h"
 
+namespace lmms
+{
+
+
 extern "C"
 {
 
@@ -244,3 +248,5 @@ PLUGIN_EXPORT Plugin * lmms_plugin_main( Model * _parent, void * _data )
 
 }
 
+
+} // namespace lmms