io/clipboard,app: add WriteOp, ReadOp

Previously, the only way to manipulate the clipboard (read or write) is
using the `app.Window`.

The new `clipboard.ReadOp` and `clipboard.WriteOp`makes possible to
read/write from the widget.

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

io/clipboard/clipboard.go

@@ -2,9 +2,36 @@
 
 package clipboard
 
+import (
+	"gioui.org/internal/opconst"
+	"gioui.org/io/event"
+	"gioui.org/op"
+)
+
 // Event is generated when the clipboard content is requested.
 type Event struct {
 	Text string
 }
 
+// ReadOp requests the text of the clipboard, delivered to
+// the current handler through an Event.
+type ReadOp struct {
+	Tag event.Tag
+}
+
+// WriteOp copies Text to the clipboard.
+type WriteOp struct {
+	Text string
+}
+
+func (h ReadOp) Add(o *op.Ops) {
+	data := o.Write1(opconst.TypeClipboardReadLen, h.Tag)
+	data[0] = byte(opconst.TypeClipboardRead)
+}
+
+func (h WriteOp) Add(o *op.Ops) {
+	data := o.Write1(opconst.TypeClipboardWriteLen, &h.Text)
+	data[0] = byte(opconst.TypeClipboardWrite)
+}
+
 func (Event) ImplementsEvent() {}