Upgrade xib resources to Xcode 5.0 format

The xib file format was modified in Xcode 5.0. We already require Xcode
5.0 so there's no reason to maintain compatibility with 4.x. Upgrade all
xib files to take advantage of the new file format in Xcode 5.0.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>