Reconcile ImageJ1 and ImageJ2 codebases

[ctrueden]

The plugin currently has separate (largely copy-pasted) implementations for ImageJ1 and ImageJ2. There should be one single implementation as a suite of ImageJ2 commands—not ImageJ1 commands. It will be possible soon to execute ImageJ2 commands from within Fiji while in legacy mode. This has the huge advantage of allowing people to start programming ImageJ2 commands and shipping them via ImageJ update sites, without users needing to "switch to modern mode" in order to run them.