Add toggle for using mirrored display as top screen.

[hjstn]

This might be useful for streamers or people recording gameplay, where both screens need to be visible. It has been tested on an iPad Pro 9.7 with X-Mirage.

This is also my first time working on/modifying an emulator’s code, so the code quality may not be great.

EDIT: I had a few freezing/crashing issues in Pokemon Black 2, but I can't tell whether it's because of this.

EDIT2: Confirmed issue and pushed fix (which required the use of KVO).