VST preset preview (#5441)

* Enable vestige presets preview.

* Don't destroy vestige instrument on every preset change.

* Don't reload VST dll plugin when it's not necessary. Always hide plugin UI in preview mode.

* Don't remove other instruments in preview mode, don't send instrument change signal.

* Minor changes

* Add a change I missed

Co-authored-by: Hyunjin Song <tteu.ingog@gmail.com>

include/InstrumentTrack.h

@@ -214,6 +214,10 @@ public:
 
 	void setPreviewMode( const bool );
 
+	bool isPreviewMode() const
+	{
+		return m_previewMode;
+	}
 
 signals:
 	void instrumentChanged();
@@ -230,6 +234,9 @@ protected:
 		return "instrumenttrack";
 	}
 
+	// get the name of the instrument in the saved data
+	QString getSavedInstrumentName(const QDomElement & thisElement) const;
+
 
 protected slots:
 	void updateBaseNote();