Display an Image's alt or title attribute in ligthbox overlay

Instead of displaying the original filename the lightbox overlay should display the alt tag and/or the title tag of the image.

The image name is often not useful at all. (E.g. something like shutterstock_1073920454325.jpg.) Futhermore the filename can’t even be changed later, since a new filename isn’t accepted, as an image already exists in the database using the same SHA1.