Move ApplicationStorageImpl files to browser folder

Move ApplicationStorageImpl files to browser folder: it should be in the same folder with ApplicationStorage.
Also ApplicationStorageImpl class should be included only in the one file - application_storage.cc, so ApplicationStorage class should use forward declaration to declare "impl" pointer.
Merge pull request #1378 from pozdnyakov/Move_Application_Storage_Impl_to_browser

Move ApplicationStorageImpl files to browser folder