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

examples/draw/main.go

@@ -34,7 +34,7 @@ func main () {
 	if err != nil { panic(err) }
 }
 
-func onPress (button stone.Button) {
+func onPress (button stone.Button, modifiers stone.Modifiers) {
 	if button == stone.MouseButtonLeft {
 		mousePressed = true
 		application.SetRune(0, 0, '+')

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)
 }
 

examples/type/main.go

@@ -55,7 +55,7 @@ func drawCaret () {
 	application.SetColor(caretX, caretY, stone.ColorDim)
 }
 
-func onPress (button stone.Button) {
+func onPress (button stone.Button, modifiers stone.Modifiers) {
 	width, height := application.Size()
 	
 	if button == stone.KeyEnter {