Correct ImageJ2Options index/hierarchy

Since there is no LegacyCompatibleCommand any more, ImageJ2Options does
not need to implement any particular interface at this time.

Also updated the plugin type to be OptionsPlugin.class, as other
OptionsPlugins.
IJ1Helper: allow special-case menu weighting

Added a getIndex method that allows for special case logic to allow the
placement of some menu items at particular positions.