Refactor out headless GenericDialog

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add rudimentary Javadoc for the headless GenericDialog

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Remove no longer needed @SupressWarnings

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a class loader specifically for loading a patched ImageJ 1.x

This class loader will only find the ImageJ 1.x classes, plus
LegacyHooks and EssentialLegacyHooks. It will also patch the ImageJ
1.x classes appropriately and install an EssentialLegacyHooks instance.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Add a "legacy environment", i.e. encapsulated ImageJ 1.x instances

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Add a test for calling a fresh headless ImageJ 1.x

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

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