merged all changes in the input branch into this one
moved Tickable to core (would have created circular library dependencies)
exported OrxListener to a separate file, soon to be deleted
changed
, &&, XOR back to or, and, xor because I found the necessary include file for VC++
created abortRequest() in Orxonox.cc to call for a smooth end of the game
(an alternative would be to make tick() return a boolean, like it is with frameStarted())