Implemented a new special-tile field that should replaced the depricated level field...

Implemented a new special-tile field that should replaced the depricated level field in world maps.
Also extended display_text_file().

special-tile can have the following flags:
 [integer] x / y - necessary to say where the tile is located
 [string] extro-filename - read the given file *
 [integer] swap-x / swap-y - say coordinates for swapping *
 [string] map-message - show a message in the world map viewer
 [string] goto-world - change the world to the given one *
 [boolean] flip-level - flip vertically the level (of course, only works when there is a level)
 [boolean] exit-game - quit game *
 [string] level - feed a level to this tile

* - if there is a level, open it only if the level is successful

SVN-Revision: 1581