TODO: Deprecate fiji-compat's User_Plugins

It served us well...

TODO: handle scripts via ij-legacy
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Retire fiji.command.Command

We kept this code only to support old code that had not been updated
in a long time. It would only compile against ImageJA, anyway, so
let's just bid the CommandListenerPlus farewell.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Bu-bye, ImageJA

From now on, we will always assume that nobody runs ImageJA anymore.

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