on_research_finished improvements

[DaveMcW]

1. on_research_finished event should specify a force.

2. force.technologies[event.research.name].researched should be set before on_research_finished is called. This makes it easier to share functions between on_research_finished and other events.

[Rseding91]

The event passes: event.research - the LuaTechnology that was researched. You can use research.force to tell what force the research is for. No need to re-lookup the technology.