Bug 338122

The limits of telescope motion should be user defined.  A mouse drawn horizon should be added, and, any slew beyond that limit should be reported as void.  Only with user confirmation should a slew outside user defined limits be allowed.  Feature operation is best observed by examining Xephem code and operation.  This visual limit on maps, greatly aids users in knowing what the telescope can, or can not, observe at a given time.  

In fixed location operation where KStars is used, obstructions such as buildings, trees, etc. render much of the displayed sky not valid.  To make the program practical, the users horizon feature allows the user to taylor KStars map display to appropriate data.