menu structure: insert separators for .jar plugins even in headless mode

ImageJ 1.x wants to make sure that plugins from .jar plugins are marked
as separate from core plugins (as if the .jar plugins would not provide
the major value of ImageJ 1.x...). So it puts separators between core
plugins and plugins inserted into the menu structure from .jar files.

Let's be faithful about that behavior when offering ImageJ 1.x' menu
structure via LegacyEnvironment.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Organize imports

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Enhance the completeness test to verify that .jar separators are
inserted

This verifies that separators are inserted into the menus before
inserting commands backed by plugin .jar files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'menu-structure-fix'

This topic branch fixes the problem when in headless mode, separators were
not inserted to make sure that users would not mistake commands backed by
plugin .jar files for core ImageJ 1.x commands.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>