Show current dB value of fader in tool tip

Show the current value of the fader in its tool tip. Please note that
this value is always shown in dB because the goal should be to get rid
of the linear values for the faders.

Some of the code is extracted into the new method
`Fader::getModelValueAsDbString` which is shared by
`Fader::modelValueChanged` (also new) and `Fader::updateTextFloat`.

Please note that `getModelValueAsDbString` will use "dB" instead of
"dBFS" as the unit and that it will format "-inf dB" instead of "-∞ dB".
These changes will also be visible in the text float that is used to
show the new values as the fader is being dragged.

include/Fader.h

@@ -125,6 +125,8 @@ private:
 	void setPeak(float fPeak, float& targetPeak, float& persistentPeak, QElapsedTimer& lastPeakTimer);
 
 	void updateTextFloat();
+	void modelValueChanged();
+	QString getModelValueAsDbString() const;
 
 	bool modelIsLinear() const { return m_modelIsLinear; }