Add creeps to scripted missions

Fixes #6298
Fixed invulnerable infantry

Closes #6263
Move common DoImpact(Target) method into DamageWarhead.
Remove more unnecessary plumbing.
Move warheads into mod code.
Convert weapon plumbing to use integer damage modifiers.
Further warhead streamlining.
Change EffectivenessAgainst to CanTargetActor.
Remove fp calculations from SpreadDamageWarhead.
Remove now-redundant AbsoluteSpreadDamageWarhead.
Convert versus to integers.
Increase falloff accuracy for large damage amounts.
Fix inner range filtering in HealthPercentageDamageWarhead.
Merge pull request #6265 from pchote/warhead-intification

Warhead cleanups
workaroud warning as error CS0067 The event is never used

closes #6101
Merge pull request #6306 from obrakmann/fix6298

Add creeps to scripted missions
Merge pull request #6313 from Mailaender/CS0067-workaround

Fixed bogus warning as error CS0067 The event is never used
Fix a bad merge in UpgradeRules.cs
Merge pull request #6303 from obrakmann/fix6263

Fixed invulnerable infantry
Refactor more power and infiltration stuff. Create new power sabotage ability for spies in ra and ts.