Implement strict typing + return types + property types on all our code

Unfortunately this is quite a big change, the important thing is to review the interfaces so that they are solid.

klausi → credited kingdutch → .

I merged this now - we can do anything that is unclear or could be improved in follow-ups. Better we get this big change out of the way now so that we can work better on other improvements.

Automatically closed - issue fixed for 2 weeks with no activity.