io/key: [API] implement key event propagation

Before this change, every Event would be passed to the focused InputOp
tag, making it impossible to implement, say, program-wide shortcuts.
This change implements key.Event routing similar to how pointer.Events
are routed: every InputOp describes the set of keys it can handle, and
the router use that information to deliver an Event to the matching
handler.

This is an API change, because every InputOp must now include a filter
matching the keys it wants to handle.

Fixes: https://todo.sr.ht/~eliasnaur/gio/395
Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/button.go

@@ -111,7 +111,7 @@ func (b *Clickable) Layout(gtx layout.Context, w layout.Widget) layout.Dimension
 	semantic.DisabledOp(disabled).Add(gtx.Ops)
 	b.click.Add(gtx.Ops)
 	if !disabled {
-		key.InputOp{Tag: &b.keyTag}.Add(gtx.Ops)
+		key.InputOp{Tag: &b.keyTag, Keys: "⏎|Space"}.Add(gtx.Ops)
 	} else {
 		b.focused = false
 	}