iOS: Add in-app display dimming to save power

On iOS, UTM disables the [Auto-Lock feature](https://support.apple.com/guide/iphone/set-a-passcode-iph14a867ae/ios#iph198262805) to prevent the device from locking and therefore stopping UTM & the virtual machine.

It would be beneficial for users on battery power to re-implement this in the app. I suggest there could be a user-configurable idle timer after which the screen brightness dims, similar to the iOS feature. Of course, the device wouldn't lock after the screen dims, that's the point.

There is a [brightness API](https://developer.apple.com/documentation/uikit/uiscreen/1617830-brightness) we can use to implement this. I'm just not sure about how to detect that the user input is idle.