Handle keyboard shortcuts in the LegacyUI

This adds special handling for the accelerators
defined by legacy-compatible Commands. Once
https://github.com/scijava/scijava-common/issues/78 is addressed, we
can change this code to handle accelerators to all legacy-compatible
Modules, not just Commands.

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