[NDS] Add button to swap screens

[BloodRaven0]

Since the second screen is generally underused in a lot of games like Castlevania, where it is only used for stuff you'd look up occasionally like enemy stats and maps, they are fully playable with only the bottom screen, however then you can't look up the map. A simple button to swap the screens would resolve this.

To clarify, this function already exists and is mapped to Right Trigger 2 when you connect a gamepad. However, it can't be accessed with the software keys, where it is even more needed, since the screen space has to accommodate the controls, along with the two screens.

So my proposal is to have an additional button that just swaps the screens, or just integrate the functionality into the existing "close lid" button, mapping the close lid function to a long-press and swap screens to a short press.