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>

src/core/ComboBoxModel.cpp

@@ -26,6 +26,9 @@
 
 #include <cassert>
 
+namespace lmms
+{
+
 using std::unique_ptr;
 using std::move;
 
@@ -69,6 +72,6 @@ int ComboBoxModel::findText( const QString& txt ) const
 }
 
 
-
+} // namespace lmms