Report fn key and lock states via HID

[Jules-Bertholet]

Fn key state is reported as "System Function Shift", FnLock key (Fn+Esc) state as "System Function Shift Lock", and Fn lock state as "System Function Shift Lock Indicator". This allows the OS to detect when the Fn key is pressed, enabling the user to configure custom keybinds involving it; it also allows the OS to report the state of the Fn lock feature to the user.

This PR, by enabling users to configure custom Fn key shortcuts, is a potential alternative to #50, #54, and especially #51.

If this PR is accepted, I plan to follow it with a patch to the Linux kernel to emit the proper input events.

Uses the same HID collection as #49; uses its branch as a base for this reason.