Package Maven Plugin: Support for package thumbnails

adding package thumbnails for the generated packages is a but cumbersome currently - one has to know exactly the internal path of the thumbnail image in the package format, and also add a ".content.xml" in the definition folder.

it would be nice to have a "thumbnailImage" parameter which accepts a path to a thumbnail PNG file - and does the correct placement in the folder and generation of the ".content.xml" then automatically.