Turn GtkShortcutTrigger into an object

Turn GtkShortcutTrigger into an object

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.