Start work on menu interactivity.

With this commit, we get a ui.Interface and ui.Widget type. The
interface monitors hover and mouse click state and tells the widgets
about them; the widgets execute code specified by the application when
events occur.

Next step: have wh40k load the main menu and play sound, etc.

cmd/view-obj/main.go

@@ -68,7 +68,7 @@ func main() {
 		lastState: state,
 	}
 
-	win, err := ui.NewWindow("View Object: " + *objName)
+	win, err := ui.NewWindow(env, "View Object: "+*objName)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -82,12 +82,12 @@ func main() {
 	win.OnMouseWheel(env.changeZoom)
 
 	// The main thread now belongs to ebiten
-	if err := win.Run(env.Update, env.Draw); err != nil {
+	if err := win.Run(); err != nil {
 		log.Fatal(err)
 	}
 }
 
-func (e *env) Update() error {
+func (e *env) Update(screenX, screenY int) error {
 	if e.step == 0 || e.lastState != e.state {
 		log.Printf(
 			"new state: sprite=%d/%d zoom=%.2f, origin=%+v",