Enforce `.DirIcon` to follow the thumnails file format specification

Doing image manipulation on `libappimage` impacts performance (measurements required) and enforce additional dependencies. This feature could be moved to `appimagetool` so every newly created AppImage will have a Thumbnailer file format specification compatible `.DirIcon` that could be simply copied into the thumbnails directory.

References:
https://github.com/AppImage/libappimage/issues/104#issuecomment-490270234