[PATCH] Minimum attack distance and spread tooltip

[elexis]

In case a unit (f.e. ranged siege, towers without murderholes) requires a minimum distance to attack enemies, this should be added to the attack tooltip (and avoided if 0).

Furthermore in Alpha 21, the spread factor has become relevant since units don't deal damage anymore if they miss (#4241) (and they miss more than 50% depending on the distance), hence should be shown.

The schema in Attack.js defines the spread:

Radius over which missiles will tend to land (when shooting to the maximum range). Roughly 2/3 will land inside this radius (in metres).

Reported by defenderbenny and Grugnas.

Patch: ​https://github.com/elexis1/0ad/tree/spread_tooltip