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>

app/window.go

@@ -168,6 +168,12 @@ func (w *Window) processFrame(frameStart time.Time, size image.Point, frame *op.
 	case router.TextInputClose:
 		w.driver.ShowTextInput(false)
 	}
+	if txt, ok := w.queue.q.WriteClipboard(); ok {
+		go w.WriteClipboard(txt)
+	}
+	if w.queue.q.ReadClipboard() {
+		go w.ReadClipboard()
+	}
 	if w.queue.q.Profiling() {
 		frameDur := time.Since(frameStart)
 		frameDur = frameDur.Truncate(100 * time.Microsecond)