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, and didn't tried to generate the full iOS project for cmake for now).
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.
Merge pull request #570 from christopho/ios

Modernize AppleInterface