exec release binding even if other keys were pressed

This commit changes the "release binding" behavior to ignore foreign
keypresses (pressed keys that are not part of a given release binding's
definition) while still triggering the bound action when the original
key from the binding is released. This, unless foreign keypresses match
a different release binding, in which case the latter binding takes
over.

For instance, assuming a release binding is configured for <Mod>, a key
corresponding to one of the possible modifier keys. The previous behavior
for the following sequence was to trigger the action only for the following
sequence:

- <Mod>(press), <Mod>(release)

but *not* in the following case:

- <Mod>(press), <Tab>(press), <Tab>(release), <Mod>(release)

With this commit both sequences trigger the bound action.

This is useful to control external software by triggering possibly
repeated actions, e.g. to navigate windows using <Mod> + <Tab>
and triggering a final action when the <Mod> key is released (e.g.
hiding a UI, reactivating LRU accounting, ...)

Closes #6456

include/sway/input/keyboard.h

@@ -65,6 +65,7 @@ struct sway_keyboard {
 	struct sway_shortcut_state state_keycodes;
 	struct sway_shortcut_state state_pressed_sent;
 	struct sway_binding *held_binding;
+	uint32_t held_keycode;
 
 	struct wl_event_source *key_repeat_source;
 	struct sway_binding *repeat_binding;