Right now there are 2 ways to center an image:

1. Put cursor in the containing paragraph and click the align button. This way the whole paragraph content, including the image, is centered.
2. Select only the image and click the align button. In this case, the image will have extra CSS class 'image-center'. This usage looks weird in editor and causes many bugs. I can understand how align left/right can be applied to image (they make the image float left/right and text floats around the image), but don't know how align center is useful to image.
   As the latter way makes no sense, we should remove it.