Change 'ini_file_name' and 'log_file_name' to accept str, bytes, or None

[KinoxKlark]

This has been issued in #290

For python users, it feels more natural to use strings to manage config and log filenames instead of bytes. I thus changed the interface of ini_file_name and log_file_name to accept str and None. It is now possible to write:

io = imgui.get_io()
io.ini_file_name = "other_name.ini"
io.ini_file_name = None
io.ini_file_name = ""

I kept the option of passing a bytes for backward compatibilities; this still works:

io = imgui.get_io()
io.ini_file_name = b'other_name.ini'

I also changed the return type of the getter to be an str instead of a bytes. This is unfortunately not backward compatible but feels more python friendly, and I don't think this is a feature that was intensively used.

[learn-more]

Makes sense 👍