Display comets at almost all zooms, default comet names now 1AU.

Previously comets were only displayed when the SkyMap was zoomed in pretty far. This changes that so that comets are displayed, and possibly "named" even when zoomed out almost all the way. There is no control for this--it was previously hard-wired to a certain zoom level, and I changed it to still be hard-wired, but to a very minimal zoom level.

Also, the default for rendering comet name labels was 3AU from the sun. I changed this to 1AU by default, though the user can change this in KStars Settings (Solar System tab).