HeadphoneKey: Start of mode handling (WIP)

A nearly-wroking stab at making mode handling work. Here we store the
current mode in the `HeadphoneKey` object. We try to save it when
getting a mode back from `c_run_key_action()`, and use it as a filter
when running `c_run_key_action()`.

I had discovered a problem, though. During debugging, I found that the
`_in_mode` ivar would end up pointing to the same thing as the current
`trigger`. Guessed that this had to do with it being a const pointer, so
I started to write some code to store the `Trigger` by value instead of
by reference in `HeadphoneKey` `_in_mode`.

At this point, a little frustrated, I thought about the overall problem
of mode handling again. It occurred to me that I no longer like this
idea of storing the current mode in the Objective-C object, as this
recent work has left a bad taste in my mouth. Instead, I'm going to try
moving mode handling into the Rust library, and make it opaque to the
Objective-C code. Hoping that that approach will be a more pleasant
endeavor.

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