Add an interface to access the IJ1 progress bar directly

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add an adapter implementing the SetProgress callback interface

The IJ1Callbacks class is intended to implement all the callback
interfaces we might want to add in the future, allowing a kind of
"duck typing" so that callers will be forward-compatible.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make the IJ1Callbacks accessible to the outside world

To obtain the active instance of the adapter, call

	env.runPlugIn(SetProgress.class.getName(), "");

That way, callers will always access the instance via the required
interface rather than by the concrete class (whose definition is most
likely not shared between caller and encapsulated ImageJ 1.x instance).

We might consider adding a convenience method to the LegacyEnvironment
to allow callers to obtain the callback object this way:

	SetProgress progress = env.getCallback(SetProgress.class);

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