Made frame dimensions equal for every image.

The frame of the selected image is now equally sized for every image,
independent of its dimensions or the existance of an exif thumbnail.
Added filenames to thumbnail view.
Fixed floating point exception in thumbnail view.

When the thumnail view was too small for for a thumnail to fit into it,
a floating point exception occurred.
Fixed size of centered images.

Images that were smaller than the size of the image view were not sized
correctly when 'centered' was set to 'false'.