app,io/system: implement clipboard reading and writing

The app.ReadClipboard and app.WriteClipboard can be used to interact
with the system clipboard. The clipboard may be asynchronous, so
system.ClipboardEvent is introduced to deliver the result of a read.

Updates gio#31

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

app/internal/window/window.go

@@ -54,6 +54,11 @@ type Driver interface {
 	// ShowTextInput updates the virtual keyboard state.
 	ShowTextInput(show bool)
 	NewContext() (Context, error)
+
+	// ReadClipboard requests the clipboard content.
+	ReadClipboard()
+	// WriteClipboard requests a clipboard write.
+	WriteClipboard(s string)
 }
 
 type windowRendezvous struct {