Major update to events and softcoding

Most ongoing and some once-off yield changes/modifiers/values will now
work for any yield.

For events the list of changes is too long to remember. Events should
now work for cities and units, tooltips for options, custom notification
text, bugfixes, items in Sets are weighted randomly chosen based on how
many modifiers they pass. There are more changes, but there's no point
in listing them all.
OnModActivated for AIMod_Defines.xml

Forgot to set OnModActivated for AIMod_Defines.xml