Improve drawing performance alot

- Don't draw a complete background gradient for departure items;
draw solid color background(s) and then draw fade out gradients at the left/right
- Hide route items if the containing departure item isn't expanded
- Store a pointer to the DepartureModel in RouteStopTextGraphicsItem,
to get it's RouteStopFlags (used for drawing)
- Still TODO: Background drawing for Journeys (just like it's done here for departures)