Rework stretch to use just MTF

stretchpanel.h

@@ -2,29 +2,21 @@
 #define STRETCHPANEL_H
 
 #include <QWidget>
-#include <QSlider>
-#include <QComboBox>
+#include "stfslider.h"
 
 class Image;
 
 class StretchPanel : public QWidget
 {
     Q_OBJECT
-    QSlider *m_lowSlider;
-    QSlider *m_highSlider;
-    QSlider *m_paramSlider;
-    QComboBox *m_stretchSelect;
+    STFSlider *m_stfSlider;
 public:
     explicit StretchPanel(QWidget *parent = nullptr);
 public slots:
     void imageLoaded(Image *img);
+    void resetMTF();
 signals:
-    void lowChanged(int low);
-    void highChanged(int high);
-    void stretchChanged(int stretch);
-    void paramChanged(float param);
-private slots:
-    void calculateParam();
+    void paramChanged(float low, float mid, float high);
 };
 
 #endif // STRETCHPANEL_H