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-map/main.go

@@ -69,7 +69,7 @@ func main() {
 		lastState: state,
 	}
 
-	win, err := ui.NewWindow("View Map " + *gameMap)
+	win, err := ui.NewWindow(env, "View Map "+*gameMap)
 	if err != nil {
 		log.Fatal("Couldn't create window: %v", err)
 	}
@@ -86,12 +86,12 @@ func main() {
 		win.OnKeyUp(ebiten.Key1+ebiten.Key(i), env.setZIdx(i+1))
 	}
 
-	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("zoom=%.2f zIdx=%v camPos=%#v", e.state.zoom, e.state.zIdx, e.state.origin)
 	}