Follow screen orientation rather than device orientation

Makes the casting orientation follow the screen orientation rather than the device orientation. For example, if an app is locked to a particular orientation then the cast will also stick to this orientation.

Matching the screen orientation feels more natural, but that could just be me. If there's uncertainty about this, we could add a configuration option to the settings.

The method I've used to determine screen orientation is really very hacky, so if there's a better way, I'd be keen to use it.