io/key: improve InputOp focus and blur

The existing implementation cannot remove the focus of some widget,
doesn't have an option to focus without display the on-screen keyboard
and it automatically focuses the first InputOp, aggressively.

That change aims to make possible: remove focus from any widget. Add
focus without displaying the on-screen-keyboard/soft keyboard. Don't
automatically focus any widget. Don't recover focus when the widget is
visible again.

Fixes gio#180.

Signed-off-by: Inkeliz <inkeliz@inkeliz.com>

widget/editor.go

@@ -399,7 +399,11 @@ func (e *Editor) layout(gtx layout.Context) layout.Dimensions {
 		e.shapes = append(e.shapes, line{off, path})
 	}
 
-	key.InputOp{Tag: &e.eventKey, Focus: e.requestFocus}.Add(gtx.Ops)
+	key.InputOp{Tag: &e.eventKey}.Add(gtx.Ops)
+	if e.requestFocus {
+		key.FocusOp{Focus: true}.Add(gtx.Ops)
+		key.SoftKeyboardOp{Show: true}.Add(gtx.Ops)
+	}
 	e.requestFocus = false
 	pointerPadding := gtx.Px(unit.Dp(4))
 	r := image.Rectangle{Max: e.viewSize}