Fix failing ScriptFinderTest

We sort the results so that the order is consistent across different
environments and operating systems. Otherwise, the order of the scripts
discovered is not predictable.

This change also renames the "Plugins" folder to "Scripts" to better
reflect the naming scheme we will actually want to test later...
ScriptService: allow menu prefix with a script dir

It may be the case that you want to set a menu path prefix to use for
all scripts within the given directory. Let's support this, particularly
for backwards compatibility with ImageJ 1.x.
ScriptFinderTest: add a test for menu prefixes
WIP: Still lots to do

- Add a unit test for ScriptService menu prefix feature
- Be smart about handling overlapping script directories
- Add a unit test for the overlapping script dir handling
- imagej-legacy: scriptService.addScriptDirectory on legacy script paths