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-minimap/main.go

@@ -71,7 +71,7 @@ func main() {
 	}
 	env := &env{gameMap: gameMap, set: mapSet, state: state, lastState: state}
 
-	win, err := ui.NewWindow("View Map " + *mapFile)
+	win, err := ui.NewWindow(env, "View Map "+*mapFile)
 	if err != nil {
 		log.Fatal("Couldn't create window: %v", err)
 	}
@@ -92,7 +92,7 @@ func main() {
 
 	win.OnMouseWheel(env.changeZoom)
 
-	if err := win.Run(env.Update, env.Draw); err != nil {
+	if err := win.Run(); err != nil {
 		log.Fatal(err)
 	}
 }
@@ -137,7 +137,7 @@ func (e *env) changeZoom(_, y float64) {
 	e.state.zoom *= math.Pow(1.2, y)
 }
 
-func (e *env) Update() error {
+func (e *env) Update(screenX, screenY int) error {
 	// TODO: show details of clicked-on cell in terminal
 
 	// Automatically cycle every 500ms when auto-update is on