allow different sight_ranges for each unit

[sanslash332]

Currently you can't have a real control of the sight range of a unit.
You can only control the range of the attack or the effect (that can be more than 12 metters (one square)) without problem, but you can't create frail units, but with a good vision, for example see up to 30 meters more than two squares in any direction, to support these units with a large attack range, but bad vision range.

So, add the possibility to control the sight range of each unit (or building)
Currentli we have the height_bonus, but that property can have other use; for example can see through walls.

I know that for now we only have paths to north, south, east and west, so can be a problem calculate the sight in diagonal; because we don't have paths on these directions.

maybe add diagonal paths for grounds units can be a good option too?

So well. Thanks for the new update; this engine is awesome.