Remove iOS debug output.

This debug output was listing the names of all the files to the iOS console
every 3 seconds. Remove it.
Fix page changing when rotating the device.

A bug in UIKit means our 'scrollViewDidScroll' method is called during
screen rotation. This ended up corrupting our current page number because
the width of the screen had changed at this point, but the pages hadn't
yet been resized/repositioned (I assume).

The workaround is to ignore calls to scrollViewDidScroll during rotation.
Fix odd animation when opening a document on iOS.

The background colour of the MuDocumentController's root view wasn't set,
so the MuLibraryController was showing through until it had been removed
from the screen.

Fix by setting the background colour to gray - the same colour as the
window.