Follow up of D2281.
- Allow techs to modify properties.
- Allow stackable.
- Ignore: Nothing happens really, the previous effect stays in effect.
- Extend: The duration of the new status effect is added to the previous.
- Replace: The previous effect is removed and the new effect is applied.
- Stack: The new effect gets a new identifier and coexists next to the previous effect.
- Fix a GUI issue where more than 5 icons would error out.
- Added test for modification.