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>

io/key/key.go

@@ -56,14 +56,6 @@ type SoftKeyboardOp struct {
 	Show bool
 }
 
-// FocusOp sets or clears the keyboard focus. It replaces any previous
-// FocusOp in the same frame.
-type FocusOp struct {
-	// Tag is the new focus. The focus is cleared if Tag is nil, or if Tag
-	// has no InputOp in the same frame.
-	Tag event.Tag
-}
-
 // SelectionOp updates the selection for an input handler.
 type SelectionOp struct {
 	Tag event.Tag
@@ -242,6 +234,13 @@ func (m Modifiers) Contain(m2 Modifiers) bool {
 	return m&m2 == m2
 }
 
+// FocusCmd requests to set or clear the keyboard focus.
+type FocusCmd struct {
+	// Tag is the new focus. The focus is cleared if Tag is nil, or if Tag
+	// has no InputOp in the same frame.
+	Tag event.Tag
+}
+
 func (k Set) Contains(name string, mods Modifiers) bool {
 	ks := string(k)
 	for len(ks) > 0 {
@@ -346,11 +345,6 @@ func (h SoftKeyboardOp) Add(o *op.Ops) {
 	}
 }
 
-func (h FocusOp) Add(o *op.Ops) {
-	data := ops.Write1(&o.Internal, ops.TypeKeyFocusLen, h.Tag)
-	data[0] = byte(ops.TypeKeyFocus)
-}
-
 func (s SnippetOp) Add(o *op.Ops) {
 	data := ops.Write2String(&o.Internal, ops.TypeSnippetLen, s.Tag, s.Text)
 	data[0] = byte(ops.TypeSnippet)
@@ -377,6 +371,8 @@ func (FocusEvent) ImplementsEvent()     {}
 func (SnippetEvent) ImplementsEvent()   {}
 func (SelectionEvent) ImplementsEvent() {}
 
+func (FocusCmd) ImplementsCommand() {}
+
 func (m Modifiers) String() string {
 	var strs []string
 	if m.Contain(ModCtrl) {