gdx-skineditor
wiki-page: https://github.com/gunmetal313/gdx-skineditor
released app: https://github.com/gunmetal313/gdx-skineditor/releases
Just unzip it and run the jar.
You have to run the jar from the command line: java -jar skin_editor_v0.31.jar
The GUI window comes up: Create your project and create the skins for your libGdx projects.