LegacyService: add getLegacyVersion() method

This method returns the version of ImageJ 1.x currently in use.
Display the version of ImageJ 1.x as well

This will only be the case when a LegacyService is successfully
initialized, of course.
ShowReadme: fix name of README file

It changed to Markdown awhile ago, but was never changed in the
ShowReadme command to match.

Of course, ImageJ still displays the Markdown-formatted text in a plain
text editor, which is a little unfortunate, but it's better than
receiving an error message as happens currently.