Scriptable abilities

**What do you want to see?**
The ability (ha!) to add scripts to abilities that run at certain points in time on all objects that have that ability, e.g. every turn.

**How would it help players?**
Modders could literally add custom abilities that actually do things and are not just flags!

**Alternatives**
N/A

**Screenshots and mockups**
N/A

**Things to consider**
When else might a script want to run, besides between turns?