HeadphoneKey: Detect 3 headphone button types

Using my previous `NSLog`s, the `usageId`s came out to be 205, 233, 234.
Converted these to hex and looked for corresponding constants in
`/System/Library/Frameworks/IOKit.framework/Versions/A/Headers/hid/IOHIDUsageTables.h`.

Use these constants to figure out which button was presed. Will have to
test other headphones. I'm worried the numbers may change for different
headphones.

Added a `KeyPress` enum that's a lot easier to understand than the
`upOrDown` `BOOL`, whose meaning I completely didn't get until I looked
at the `NSLog` output I had before.

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