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/menus/menus.go

@@ -170,29 +170,6 @@ func (r *Record) Toplevel() *Record {
 	return r
 }
 
-func (r *Record) SelectSprite(step int, pressed, focused bool) int {
-	switch r.Type {
-	case TypeStatic:
-		return r.SpriteId[0]
-	case TypeMenu:
-		return r.SpriteId[0] // Probably -1
-	case TypeOverlay:
-		return r.Share
-	case TypeMainButton:
-		// A main button has 4 states: unfocused, focused (animated), mousedown, disabled
-		if focused && pressed {
-			return r.Share + 1
-		} else if focused {
-			return r.SpriteId[0] + (step % r.DrawType)
-		}
-
-		// TODO: disabled
-		return r.Share
-	}
-
-	return -1
-}
-
 func setProperty(r *Record, k, v string) {
 	vSplit := strings.Split(v, ",")
 	vInt, _ := strconv.Atoi(v)