DefaultScriptService: remove superfluous javadoc
ScriptService: fix scripts path system property

It was an error that the prefix was still "ij." after migrating from
ImageJ. It should be "scijava." instead.
ScriptService: add API to add & remove script dirs

We want downstream services to have the opportunity to amend the list of
directories from which scripts should be discovered. In particular,
ImageJ's LegacyService will a couple of additional folders for reasons
of backwards compatibility with ImageJ 1.x.