Is your feature request related to a problem? Please describe.

There is a reference from hwIo.base.IoBase to braille._BgThread that we need to get rid of at some point. This will probably be API breaking, but it would really help if we could make IoBase independent of the background thread that it runs it APCs on.

Describe the solution you'd like

Provide a callable to the constructor of IoBase where it can throw its APC at.