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-set/main.go

@@ -51,11 +51,6 @@ func main() {
 		log.Fatalf("Couldn't load set %s: %v", *setName, err)
 	}
 
-	win, err := ui.NewWindow("View Set: " + *setName)
-	if err != nil {
-		log.Fatal("Couldn't create window: %v", err)
-	}
-
 	state := state{zoom: 8.0}
 	env := &env{
 		set:       set,
@@ -63,6 +58,11 @@ func main() {
 		lastState: state,
 	}
 
+	win, err := ui.NewWindow(env, "View Set: "+*setName)
+	if err != nil {
+		log.Fatal("Couldn't create window: %v", err)
+	}
+
 	win.OnKeyUp(ebiten.KeyLeft, env.changeObjIdx(-1))
 	win.OnKeyUp(ebiten.KeyRight, env.changeObjIdx(+1))
 
@@ -72,12 +72,12 @@ func main() {
 	win.OnMouseWheel(env.changeZoom)
 
 	// 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 {
 	curObj, err := e.curObject()
 	if err != nil {
 		return err