[WIP] Add type hints to gramps.gen

[DavidMStraub]

This is work in progress adding type hints to gramps.gen and defining Gramps-specific types in the process.

[dsblank]

@DavidMStraub, one complication is that when these methods are overloaded in a proxy (like LivingProxyDb, or PrivateProxyDb) then a method might return None. This is the number one issue in bugs throughout the code base: we don't check to see if the return value is None and then we end up with crashes.

I think with the correct type hints, we could catch such errors.

[DavidMStraub]

Exactly! I have been bitten by this before. I suspect there are similar, more hidden cases.