Explore scripts in projects

[ap--]

For a few specific UI things it might be nice if paquo offered a way of populating the scripts folder.
minimal implementation would just be:

# this is not implemented yet
QuPathProject.add_script(name, groovy_txt)

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So with per project startup.groovy files (ref: required qupath startup.groovy), this could become a useful thing if we prepare projects for users.