* Ensure that modifier keys are identified even when the next key does
not produce a keysym. This requires that modifier change tracking
be done for each sway_shortcut_state.
* Permit regular and --release shortcuts on the same key combination.
Distinct bindings are identified for press and release cases; note
that the release binding needs to be identified for both key press
and key release events.
* Maintain ascending sort order for the shortcut state list, and keep
track of the number of pressed key ids, for simpler (and hence
faster) searching of the list of key bindings.
* Move binding duplicate detection into get_active_binding to avoid
duplicating error messages.
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