Allow setting signatures via decorators

[luigi311]

Is your feature request related to a problem? Please describe.
Right now we have to use 's' for return types but this causes issues with mypy due to unknown type errors and it causes it to get highlighted in editors with mypy support.

Describe the solution you'd like
A solution could be to instead use a decorator to set the signatures so we can use normal python types in the typing system.

Additional context
This was originally drafted in python-dbus-next here altdesktop/python-dbus-next#111 for some more information

[bdraco]

Seems like a reasonable change if you are considering a PR