Allow user to override button title on split views

This makes it so that the user can override the button title on split views when there's a button in portrait mode. It used to always take the master view's title, but now the user can specify `:button_title` when creating the split screen to permanently override that setting.

```ruby
open_split_screen main_screen, detail_screen, button_title: "Something Different"
```

With passing test :grin: