Add “Overwatch” attack context for ranged attacks

[dixhuit]

This would appear as a checkbox next to “Defender in cover” etc in the “Attack context” panel (for ranged attacks only). Selecting this checkbox would override the requirement to hit to be an unmodified 6.

Also add "Overwatch" as a new modifier effect where we can override the default success roll value to be something other than 6.

https://www.reddit.com/r/UnitCrunch/comments/15q0d1r/overwatch_as_preset_for_global_modifiers/

[dixhuit]

Another request for a similar thing: https://www.reddit.com/r/UnitCrunch/comments/1biypap/how_to_test_fire_overwatch_critical_hits_torrent/