Fix a couple of compiler warnings in fiji-compat's IJ1 hacking code

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make sure that ImageJ2's legacy layer gets initialized

... before we start up ImageJ 1.x. This is needed for the upcoming
Help>Switch To Modern Mode.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
IJHacker: be nice if ij-legacy already defined some classes

We try to reconcile the Javassist-based IJ1 patching between ImageJ2
and Fiji, but for the time being, let ImageJ2 do its thing after
Fiji. That means that we only should make sure that the patched classes
are defined after ImageJ2 did its patching (and may have defined said
classes including Fiji's patches). So be prepared...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add ij-app as a dependency of fiji-compat

This is needed for the upcoming Help>Switch to Modern Mode command.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add Help>Switch To Modern Mode

Based on the recent improvements in ij-legacy.

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
JavassistHelper: do not rely on isFrozen

When classes are loaded via ClassPool#toClass, they are marked as
frozen, but not their dependnecy classes (even if they were modified
using Javassist, too). Hence we need to be very lenient when it comes
to defining classes after giving ij-legacy a shot.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fiji-compat's Mighty Mouse handling moved to ij-legacy

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fiji-compat's CompositeImage handling moved to ij-legacy

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fiji-compat's ImagePlus handling moved to ij-legacy

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Some of fiji-compat's ij.IJ handling moved to ij-legacy

... but only the non-Fiji-specific parts.

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