Visitor design pattern implemented for WorldBlock

Track, Junction and Station all can now be identified
without using RTTI. Useful for any class with a need
to differentiate behavior according to WorldBlock's
subtype. More information on Visitor can be found here:

http://staff.elka.pw.edu.pl/~rnowak2/dyd/zpr2013Z/wykl04-virtual.pdf
starting from page 21 onwards.