Add LegacyCompatibleCommands to IJ1 menus

The LegacyCompatibleCommand is now a marker interface to designate IJ2
commands that should be added to the ImageJ1 menu structure.

The DefaultLegacyService will now find these commands and, preserving
their menu structure, insert them into the ImageJ1 menus. Existing,
overlapping menu structure will be reused where avilable, and created if
it does not already exist.