Custom Warheads refactor

Changes included:

Warhead code split out of weapon code and refactored.
Warhead functionality now split into several classes, each handling one effect/impact.

Additional custom warheads can now be defined and called via yaml.
Custom warheads inherit the abstract class Warhead,
which provides target check functions.

Custom warheads have to define their own impact functions,
and can also define their own replacement for check
functions.
Adds Diplomacy keywords for ValidTarget checks.

Squashed commit:

Allows diplomacy stance checks to be done on warheads and weapons, using keywords in ValidTargets and InvalidTargets.
Rebase.