Use dbFS scale for the faders

Make the faders use a linear dbFS scale. They now also change position on first click and can then be dragged.

The `Fader` class now has a new property called `m_faderMinDb`. It's the minimum value before the amplification drops down fully to 0. It is needed because we cannot represent a scale from -inf to maxDB.

Rename `knobPosY` to `calculateKnobPosYFromModel` and move the implementation into the cpp file. The method now first converts the model's current amplification value to dbFS and then computes a ratio based on that values and the minimum and maximum dbFS values.

Add the method `setVolumeByLocalPixelValue` which takes a local y coordinate of the widget and sets the amplification of the model based on a dbFS scale.

Adjust `mousePressEvent` and `mouseMoveEvent` so that they mostly take the local y coordinate of the event and use that to adjust the model via `setVolumeByLocalPixelValue`.

Remove `m_moveStartPoint` and `m_startValue` and they are not needed anymore.

include/Fader.h

@@ -119,13 +119,8 @@ private:
 
 	void paintLevels(QPaintEvent* ev, QPainter& painter, bool linear = false);
 
-	int knobPosY() const
-	{
-		float fRange = model()->maxValue() - model()->minValue();
-		float realVal = model()->value() - model()->minValue();
-
-		return height() - ((height() - m_knob.height()) * (realVal / fRange));
-	}
+	int calculateKnobPosYFromModel() const;
+	void setVolumeByLocalPixelValue(int y);
 
 	void setPeak(float fPeak, float& targetPeak, float& persistentPeak, QElapsedTimer& lastPeakTimer);
 
@@ -147,8 +142,8 @@ private:
 
 	bool m_levelsDisplayedInDBFS {true};
 
-	int m_moveStartPoint {-1};
-	float m_startValue {0.};
+	// The dbFS amount after which we drop down to -inf dbFS
+	float const m_faderMinDb {-60.};
 
 	static SimpleTextFloat* s_textFloat;