Add support for script editor templates

We had some dysfunctional code for populating the Template menu, inherited from when the script editor was maintained inside Fiji. This code inspected only the .jar file in which the editor was defined, though.

Much better to do this differently! Our new support now looks into *all* `META-INF/script-templates/` directories of all the class path elements. That way, the templates can be shipped together with the language, or with the plugin that supports the called API.