Modifiers were moved into Key in 22169e2, but the separate Modifier field
on types.Binding and gocui.keybinding was left behind. The keypress matcher
already compares modifiers via Key.Equals, so the old field is never read on
the dispatch path; it just got passed through SetKeybinding and stored.

Drop it from gocui.keybinding, types.Binding, and the SetKeybinding /
DeleteKeybinding signatures, and remove every now-redundant Modifier:
gocui.ModNone struct field. Mouse bindings keep their own Modifier (on
ViewMouseBinding) since that path still consults it.