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
ScriptFinder: fix section header
ScriptFinder: discover each script only once

Even if overlapping base script directories are added, such that a
script might be examined more than once, let's only add it to the first
time it is matched.

This will be useful to support Fiji 1.x's special handling of scripts in
the Plugins/Scripts subtree.
ScriptFinderTest: add test for overlapping dirs
ScriptFinderTest: avoid emitting warnings

The default scripts directory is not actually present, so let's exclude
it from our search. This avoids warnings about nonexistent scripts
directories.
Merge branch 'overlapping-script-dirs'

This branch improves the script finder logic:
- Adds API to specify a menu prefix for each base script directory.
- Adds each script only once, even if present in script directories.
- Adds unit tests for these features.