io/input,io/key: [API] introduce Command, replace FocusOp with FocusCmd

Modeling focus change as an operation is awkward, because focus changes
logically happen during event processing, not layout. In particular, you
want to apply focus changes even if a widget is subsequently never laid
out.

Now that input.Source is concrete, it's much more straightforward to
offer focus changes as a command which can be queued through the
Source. A future change may similarly offer a command for directional
focus changes.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/selectable.go

@@ -210,7 +210,7 @@ func (l *Selectable) Layout(gtx layout.Context, lt *text.Shaper, font font.Font,
 	}
 	key.InputOp{Tag: l, Keys: keys}.Add(gtx.Ops)
 	if l.requestFocus {
-		key.FocusOp{Tag: l}.Add(gtx.Ops)
+		gtx.Queue(key.FocusCmd{Tag: l})
 		key.SoftKeyboardOp{Show: true}.Add(gtx.Ops)
 	}
 	l.requestFocus = false