Add named object index and improve ObjectService

[imagejan]

Fixes #371.

[imagejan]

This change to DefaultWidgetModel is probably superfluous. I figured that dropdown choices such as for images and other objects will be rendered by ObjectWidget and require changing the renderer for list cells as done in scijava/scijava-ui-swing@2411016.

The getChoicesmethod of WidgetModel seems to be exclusively used for the choices attribute of Parameters, right? What confused me was the generic typing List<?> of item.getChoices().

In summary, I am not sure whether we should change this line to use ObjectService, or leave it unchanged.

[ctrueden]

Intuitively, using the ObjectService seems less hacky, so let's change it for now, and revert if any problems arise.

[imagejan]

Judging from:

scijava-common/src/main/java/org/scijava/module/AbstractModuleItem.java

Lines 276 to 280 in 9231219

	@Override
	public List<T> getChoices() {
	final T[] choices = getType().getEnumConstants();
	return choices == null ? null : Arrays.asList(choices);
	}

)

... the ModuleItem#getChoices() seems to be used for enums, other than for String choices.

But I didn't find an actual application case where the choice widget is used when and enum parameter is requested. Is there any?

In any case, the above line will probably never be called on Named objects or on objects registered to the ObjectService, so I'll remove it in a follow-up commit.