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.

internal/wh40k/wh40k.go

@@ -27,5 +27,7 @@ func Run(configFile string) error {
 	wh40k.PlaySkippableVideo("LOGOS")
 	wh40k.PlaySkippableVideo("movie1")
 
+	// TODO: load main interface
+
 	return nil
 }