Added a global Do function

tomo.go

@@ -35,6 +35,10 @@ func Stop () {
 	backend = nil
 }
 
+func Do (callback func ()) {
+	backend.Do(callback)
+}
+
 func NewWindow (bounds image.Rectangle) (MainWindow, error) {
 	assertBackend()
 	return backend.NewWindow(bounds)