add covering type hints

[toofar]

Not all code necessarily needs to be type hinted, but that is the preference. There are some files in the repository that currently require 100% coverage for type hints. There is some guidance in the contributor docs: https://qutebrowser.org/doc/contributing.html#:~:text=Type%20hinting:

There is also some places where type hints have been added or changed that need reviewing. For example does MainWindow.tabbed_browser need to be a union when TreeTabbedBrowser is supposed to be a subclass of TabbedBrowser?

[thenightmail]

I'm interested in this. Some of the type hints seem easier than others to implement. My experience with python and github is limited. Guess I'll focus on the easier ones first. Thanks for the opportunity.