run_key_action_for_mode(): Keep track of `in_mode` in `state`

Instead of letting the Objective-C code keep track of `in_mode` by
passing it around in `CKeyActionResult`, keep track of it in a `State`
struct.

Derive `Clone` on `HeadphoneButton` to resolve this error:

    error[E0277]: the trait bound `parser::HeadphoneButton: std::clone::Clone` is not satisfied
       --> src/cocoa_bridge.rs:273:38
        |
    273 |         state.in_mode = Some(trigger.to_vec());
        |                                      ^^^^^^ the trait `std::clone::Clone` is not implemented for `parser::HeadphoneButton`

1 files changed, 1 insertions, 1 deletions

diff --git a/dome_key_map.h b/dome_key_map.h
index 633e9b3..fc2e7fd 100644
--- a/dome_key_map.h
+++ b/dome_key_map.h
@@ -36,7 +36,7 @@ typedef struct {
   const Trigger *in_mode;
 } CKeyActionResult;
 
-const CKeyActionResult *c_run_key_action(Trigger trigger, const Trigger *mode);
+const CKeyActionResult *c_run_key_action(State *state, Trigger trigger, const Trigger *mode);
 
 void state_free(State *ptr);