now sight to an adjacent square is blocked if there is no exit

Notes:
- air units, towers and units on higher ground are not affected
- diagonals don't work for units at the same altitude (this might be fixed soon).

Additional changes:
- completely removed "sight_range" as a rules.txt property
- added a "height_bonus" property for ground units like towers (or giants, maybe)
- completely removed "special_range": use "minimal_range" and "range" instead (example: catapult)
- made sure that "minimal_range" is internally a "precision" property
- fixed one test to take diagonals into account
bug fixed: minimal_range wasn't implemented