Added a rudimentary mechanism for reacting to messages from the host

[SteveBenz]

This provides a mechanism for reacting to messages from the host to the HID device. It's not a particularly lovely solution, but nicer mechanisms for callbacks (e.g. virtual methods, functors) would struggle with back compat. I'm not anything like an expert in this space, but I believe that HID devices really don't have a need for much more than what I've got here. It's enough to be able to do stuff like set keyboard LED's.

Fixes: #6390

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