Basic SplitScreen functionality for iPad

Create a split screen with split_screen(). This function give back the SplitViewController.

Example:
```ruby
open_tab_bar(
  split_screen(
    screen1: ListScreen.new(nav_bar: true, title: "List"),
    screen2: DetailScreen.new(nav_bar: true, title: "Details"),
    icon: "myicon", title: "Splittest"
  ),
  OptionsScreen.new(nav_bar: true)
)
```

For change the detail screen use open as normal.