Add support for StorageItemAccessList for accessing files outside of WinRT sandbox

Currently the file and folder pickers for WinRT/Windows Phone do not provide means to keep file/folder access permissions after application restart. This is a big bummer for WinRT/WinPhone applications.

WinRT provides StorageItemAccessList/FutureAccessList functionality to keep the permissions. The number of permissions is currently limited by 1000.

Each file permission is referenced by a string token. So we either need to get access to the IStorageFile/IStorageFolder returned from the picker dialog (QFileDialog) and manually extract the token in the application, or provide an argument to add the picked file/folder to the FutureAccessList and return the string token.

Further it is required to somehow get a QFile from the token.

[1] https://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.accesscache.storageitemaccesslist