Modernize AppleInterface

Use Objective-C++ functions and @autoreleasepool instead of manually setting a pool.
It allow to build Solarus on iOS natively (at least by copying source files into a new XCode iOS project, I didn't tried to generate the full iOS project with cmake for now, even if it should works).
The modernization was needed because of the new ARC system with Obj-C objects, which wasn't supported before since the NSAutoReleasePool was explictely used.