Apply curve to faders

Apply a curve, i.e. the cube function and its inverse, to the fader
positions to that we have more space to work with in the "interesting"
areas around 0 dB and less space in the area where we tend to "-inf dB".

Set the minimum dB value of the fader to -120 dB to increase the potential
headroom.

include/Fader.h

@@ -143,7 +143,7 @@ private:
 	bool m_levelsDisplayedInDBFS {true};
 
 	// The dbFS amount after which we drop down to -inf dbFS
-	float const m_faderMinDb {-60.};
+	float const m_faderMinDb {-120.};
 
 	static SimpleTextFloat* s_textFloat;