IJ1Helper: only set main window title and icon if there is a main window

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
IJ1Helper: be more careful when adding menu items

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Invert the LegacyCompatibleCommand logic

Instead of having to mark commands that are legacy-compatible, we now
require that commands that want to be marked as legacy-incompatible
*explicitly* by adding a 'no-legacy' attr.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
IJ1Helper: Allow modern commands to override legacy ones

Unless a command is marked explicitly with a 'no-legacy' Attr, it
should be allowed to override existing legacy commands.

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

It is no longer needed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Remove the legacy-compatible editor plugin

We just inverted the logic with regards to legacy-compatible commands:
instead of marking the compatible ones, we require the "no-legacy" attr
in the @Plugin annotation for the *incompatible* ones instead.

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