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/DynamicsProcessorControlDialog.cpp

@@ -32,6 +32,9 @@
 #include "Knob.h"
 #include "PixmapButton.h"
 
+namespace lmms::gui
+{
+
 
 DynProcControlDialog::DynProcControlDialog(
 					DynProcControls * _controls ) :
@@ -151,3 +154,5 @@ DynProcControlDialog::DynProcControlDialog(
 			_controls, SLOT( subOneClicked() ) );
 }
 
+
+} // namespace lmms::gui