Enable antialiasing in the plot widgets of the Alt vs Time and Solar System tools.

Also improved the twilight gradients, using QLinearGradient instead of repeated calls to drawLine while stepping in color :)

I noticed that with antialiasing active, a horizontal or vertical line drawn with integer pixel values is drawn "fuzzy" and half-transparent.
I offset the coordinates of the yellow lines drawn at mouse click positions by 0.5 pixels to sharpen them up. This works really well.

I should apply the same sharpness fix to KPlotWidget's axis lines.