IJ1Helper: simplify log statements
IJ1Helper: add missing javadoc
IJ1Helper: make getVersion() method more robust

ImageJ1 should not use a public string constant. Those get inlined.

Let's not fall into the trap of using it.
http://javasplitter.blogspot.com/2011/10/static-final-inline-trap.html
DefaultLegacyHooks: fix javadoc and comments
LegacyService: add getCombinedVersion() method

This method returns the ImageJ version string as the IJ2 version,
slash-separated from the IJ1 version.
DefaultLegacyHooks: get version from LegacyService

We can lean on the new LegacyService#getCombinedVersion() method when
deciding what version string to use in the ImageJ 1.x status bar.