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/SpectrumAnalyzer/SaSpectrumView.cpp

@@ -43,6 +43,9 @@
 	#include <chrono>
 #endif
 
+namespace lmms::gui
+{
+
 
 SaSpectrumView::SaSpectrumView(SaControls *controls, SaProcessor *processor, QWidget *_parent) :
 	QWidget(_parent),
@@ -843,3 +846,5 @@ void SaSpectrumView::resizeEvent(QResizeEvent *event)
 	m_linearAmpTics = makeLinearAmpTics(m_processor->getAmpRangeMin(), m_processor->getAmpRangeMax());
 }
 
+
+} // namespace lmms::gui