Repeated keys are detected properly

The repeated bool was removed and instead key release events are
*only* sent when the key is actually let go. If an element wants to
listen to repeat presses, it can just listen to press events.

elements/basic/checkbox.go

@@ -70,11 +70,7 @@ func (element *Checkbox) HandleMouseUp (x, y int, button tomo.Button) {
 func (element *Checkbox) HandleMouseMove (x, y int) { }
 func (element *Checkbox) HandleMouseScroll (x, y int, deltaX, deltaY float64) { }
 
-func (element *Checkbox) HandleKeyDown (
-	key tomo.Key,
-	modifiers tomo.Modifiers,
-	repeated bool,
-) {
+func (element *Checkbox) HandleKeyDown (key tomo.Key, modifiers tomo.Modifiers) {
 	if key == tomo.KeyEnter {
 		element.pressed = true
 		if element.core.HasImage() {