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/PeakControllerEffectControlDialog.cpp

@@ -35,6 +35,8 @@
 #include "LedCheckBox.h"
 #include "embed.h"
 
+namespace lmms::gui
+{
 
 PeakControllerEffectControlDialog::PeakControllerEffectControlDialog(
 				PeakControllerEffectControls * _controls ) :
@@ -104,3 +106,5 @@ PeakControllerEffectControlDialog::PeakControllerEffectControlDialog(
 	setLayout( mainLayout );
 }
 
+
+} // namespace lmms::gui