Applying patch to improve rendering time for stars by a factor of twenty. Stars are now drawn as pixmaps, rather than calling drawEllipse() for each star. The star pixmaps are stored in a static QHash in StarObject. The hash contains images for all spectral types, and all sizes. This is how stars were rendered in 3,5, so this is a regression fix.

The star pixmaps are regenerated when the user selects a new color saturation level for stars (this is effected by modifying the width of the QPen used to draw the colored rim of the circle), and when the user chooses a color scheme that uses solid red, black or white star images (without a colored rim).

Also, stars are now rendered slightly larger on-screen, closer to what was done in 3.5.