Add proper and reliable (i hope) support for modifier keys

examples/printbuttons/main.go

@@ -32,7 +32,7 @@ func main () {
 	if err != nil { panic(err) }
 }
 
-func onPress (button stone.Button) {
+func onPress (button stone.Button, modifiers stone.Modifiers) {
 	println("press", button)
 }