implement file picker in iOS with .ext to UTI filtering

PanoPainter/GameViewController.h

@@ -16,19 +16,24 @@
 @interface GameViewController : UIViewController<
     UIKeyInput,
     UIImagePickerControllerDelegate,
+    UIDocumentPickerDelegate,
     UINavigationControllerDelegate,
     UIDocumentInteractionControllerDelegate
     >
 {
 @public GameView* glview;
 @public EAGLContext* context;
+@public bool input_enabled;
 }
+- (void)show_keyboard;
+- (void)hide_keyboard;
 - (void)display_file:(std::string)filename;
 - (void)reset_touch;
 - (void)async_lock;
 - (void)async_unlock;
 - (void)async_swap;
 - (void)pick_photo:(std::function<void(std::string)>) callback;
+- (void)pick_file:(NSArray<NSString*>*)types then:(std::function<void(std::string)>)callback;
 - (void)registerForKeyboardNotifications;
 - (void)unregisterForKeyboardNotifications;
 - (void)crash;