add changelog dialog and display_file api (implemented in osx and ios)

PanoPainter/GameViewController.m

@@ -62,6 +62,21 @@ NSThread* lock_thread;
     [self.context presentRenderbuffer:GL_RENDERBUFFER];
 }
 
+-(void)display_file:(std::string)filename
+{
+    NSString* filePath = [NSString stringWithUTF8String:filename.c_str()];
+    NSURL *url = [NSURL fileURLWithPath:filePath];
+    UIDocumentInteractionController *popup = [UIDocumentInteractionController interactionControllerWithURL:url];
+    [popup setDelegate:self];
+    [popup presentPreviewAnimated:YES];
+}
+
+-(UIViewController *)documentInteractionControllerViewControllerForPreview:(UIDocumentInteractionController *)controller
+{
+    return self;
+}
+
+
 - (void)pick_photo:(std::function<void(std::string)>) callback
 {
     GameImagePicker *picker = [[GameImagePicker alloc] init];