Generalize effect ranges

[lmoureaux]

Is your feature request related to a problem? Please describe.
Currently, effect ranges are limited to a few predefined values.

Describe the solution you'd like
Allow context-dependent expressions such as attacker.home_city for a combat-related effect.

Describe alternatives you've considered
Write all effects in Lua. Allowing a Turing-complete syntax would make them much harder to document automatically and would make the AI's task close to impossible.

Additional context
Would allow replacing targets= and many other things.