GFX: Add support for loading external images

[lcdr]

GFX files have the ability to reference external images through the DefineExternalImage tag. In the current FFDec, this tag is only parsed, but does not have image functionality like the other image tags. Also, the parsing is incorrect, with FFDec missing a string in the tag. This PR fixes the parsing and adds support for loading external DDS images via DefineExternalImage tags.

In order to support the loading of DDS files, the library DDSReader (licensed under MIT) is added as a dependency. Using this library, the referenced images are read to a BufferedImage, scaled to their target dimensions, and stored as a SerializableImage. Images can be replaced with the usual file picker dialog, however changes cannot currently be saved as the setImage function does not have access to the file path which would be needed to reference an external image (and silently changing the tag type to an internal image seemed to me like the wrong way of doing things).

I wasn't sure whether I should add an entry for DDS files in the format enum, so the image format is reported as PNG for now.

This is my first time contributing to this codebase, so I'm not sure whether I did everything the right way. I can make changes if requested.

Examples:

Before:

After:

Minimal testcase:
minimal.zip

[jindrapetrik]

Thank you very much, you did a great job, even with samples! Cool.
This is very good pull request, I think it has everything it should have.