Add DragCommand. Default Y/N on optional fields. Open/Save project items.

- More Y/N in Optional, making it easier to omit parameters.
- AT removed from Envelope, since T already gives it.

src/TrackPanel.cpp

@@ -2197,6 +2197,8 @@ void TrackInfo::GainSliderDrawFunction
    auto target = dynamic_cast<GainSliderHandle*>( context.target.get() );
    auto dc = &context.dc;
    bool hit = target && target->GetTrack().get() == pTrack;
+   if( hit )
+      hit=hit;
    bool captured = hit && target->IsClicked();
    SliderDrawFunction<WaveTrack>
       ( &TrackInfo::GainSlider, dc, rect, pTrack, captured, hit);