several minor bugfixes:  circumpolar objects no longer cause error message when popup menu is invoked.  modified slewing behavior near zenith and nadir in altaz mode.  tweaked checkVisibility function to make sure all onscreen objects get drawn.  adjusting timestep automatically returns input focus to skymap.  Setting Lat=90.0 degrees (i.e., Norht pole) breaks display; maximum allowed latitude is now 89.9999.  other minor tweaks.

svn path=/trunk/kdeedu/kstars/; revision=134929