We no longer need to hold explicit references to event subscribers

... except when we need to unsubscribe them explicitly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
SwingScriptRecorder: remember the event subscribers

We need to unsubscribe them after recording. That code is there. But
the subscribers were never set...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'weak-event-subscribers'

With scijava-common 1.8.0, EventService.subscribe() no longer forces
the callers to hold a reference to the callers anymore.

As ImageJ2 serves also as documentation how to do things, let's
stop holding references where possible (i.e. where we do not want to
unsubscribe the event listeners explicitly later).

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