showbase: Annotate basic ShowBase setup/shutdown methods

[WMOkiishi]

Change description

These changes add type annotations to some basic ShowBase methods used on setup and shutdown. This mostly comprises adding annotations to attributes assigned in ShowBase.__init__. See also #1217, which aimed to do mostly the same thing, but seems to have been abandoned.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[WMOkiishi]

Alright, I think I've found a good way to make mypy not complain about setting custom attributes on builtins. Let me know if it's acceptable.