C# Класс Bloom.Edit.PageEditingModel

Check whether the new image file is the same as the one we already have chosen. (or at least the same pathname in the filesystem)

See https://silbloom.myjetbrains.com/youtrack/issue/BL-2776. If the user goes out of his way to choose the exact same picture file from the original location again, a copy will still be created with a slightly revised name. Cropping a picture also results in a new copy of the file with the revised name. We still need a tool to remove unused picture files from a book's folder. (ie, BL-2351)