FR: Support multiple selection in iOS native FileChooser (pull request included)

I think it can be useful to batch import presets, samples, etc.
This has been a feature since iOS 11, but still hasn’t made it into JUCE. Moreover, the old -documentPicker:didPickDocumentAtURL: method of UIDocumentPickerDelegate is deprecated - again, since iOS 11.
The implementation falls back to single selection for earlier iOS versions.