EssentialLegacyHooks: ensure correct context class loader when initializing

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
LegacyHooks: implement all methods

LegacyHooks is still an abstract class because it cannot do anything
interesting due to the contract of not using any ImageJ 1.x classes.
However, it is a good idea to make it easier to extend by providing
dummy functionality for all methods.

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