Add python type hints

Is your feature request related to a problem? Please describe.

Type hints and type checkers using them has grown and shown to be really good at catching errors early, and at providing info to IDEs. In Kivy ecosystem I only found https://github.com/tonyfinn/mypy-types-kivy, an old effort that could only loose sync by living in a separate repo.

It would be really useful to be able to use those type checkers on code using Kivy, but they can only do limited checks when the libs does not provide typing information.

Would this topic be open to PR?

Describe the solution you'd like

Starting to add type hints to the API, and a export py.typed file to advertise type support.

There may be some limitation when it comes to parameters passed as kwargs, but that can be left for later, any bit of a type hint is already useful.

Describe alternatives you've considered

Separate repo like https://github.com/tonyfinn/mypy-types-kivy, but that really only makes sense for non python-native libraries.