gui: PreviewGenerator: Add get()

gui/previewgenerator.h

@@ -2,6 +2,7 @@
 #define PREVIEWGENERATOR_H
 #include <QVector>
 #include <QSharedPointer>
+#include <QFileInfo>
 #include "previewresult.h"
 #include "renderconfig.h"
 
@@ -12,6 +13,8 @@ class PreviewGenerator
 	virtual ~PreviewGenerator()
 	{
 	}
+
+	static PreviewGenerator *get(QFileInfo &info);
 };
 
 #endif // PREVIEWGENERATOR_H