The lightweight inheritance mechanism used for GtkShortcutTrigger is not
going to be usable by bindings, because boxed types cannot have derived
types.
We could use GTypeInstance and derive everything from that, like
GParamSpec, but in the end shortcuts are not really a performance
critical paths, unlike CSS values or render nodes.
Copy the logic from GtkKeyHash for matching key events
to shortcuts.
Adapt shortcuts test to work with the better matching,
by creating more complete key events.
Allow GtkShortcutTrigger to return partial matches.
Currently, no triggers produce such results, and
GtkShortcutController treats partial matches like
exact ones.
