Remove Images directory

The code in the Images directory is not being developed and it is in very bad shape. The only other code in TNL using this are the tnl-dicom-reader and tnl-image-converter tools, which are very limited compared to e.g. ImageMagick and various Python libraries/scripts. While the code in Images may be useful for some applications built on top of TNL, there are many image processing libraries that could be used instead. So I propose to remove this code – it can be backed up in the tnl-legacy repository if it's needed in the future.