Enter fader value when space key pressed

Let the users enter the fader value via dialog when the space key is
pressed.

Extract the dialog logic into the new method `adjustByDialog` and call
it from `MixerView::keyPressEvent`.

Make `Fader::mouseDoubleClickEvent` delegate to `adjustByDialog` and
also fix the behavior by accepting the event.

include/Fader.h

@@ -102,6 +102,8 @@ public:
 	void adjust(const Qt::KeyboardModifiers & modifiers, AdjustmentDirection direction);
 	void adjustByDecibelDelta(float value);
 
+	void adjustByDialog();
+
 	void setDisplayConversion(bool b)
 	{
 		m_conversionFactor = b ? 100.0 : 1.0;