introspection: Add more filename type annotations for strings which can contain filenames

This continues the changes done in https://bugzilla.gnome.org/show_bug.cgi?id=767245

This makes it possible to pass Python path types as process arguments and env vars
in PyGObject and and makes it clear that the values are not strictly utf-8 and need
to be validated/converted first.

https://bugzilla.gnome.org/show_bug.cgi?id=788863