ClassUtils: add some explanation about performance

We may not _always_ need to include interfaces when recursively scanning
for annotated methods, but for now it does not seem worth worrying over.
ServiceHelper: load EventService more aggressively

Whenever we have @EventHandler annotated methods, we need an
EventService to be loaded as a dependency. Otherwise, even if the
EventService is loaded later, those event handlers will not be properly
registered with it, since at the time registration was attempted, the
EventService was not yet loaded and hence null.

This change obviates the need for DefaultEventService being set to an
absurdly high priority, as explained at:

https://github.com/imagej/imagej/commit/b9abe178b813fb68aa49fdd7515a5e6698436793
EventServiceTest: test support for @EventHandler

Specifically, we test that when one or more @EventHandler-annotated
methods are present in a service, that it gets interpreted as a
dependency on EventService, even if there is no @Parameter-annotated
EventService field.
Context: remove obsolete warnings from javadoc

Now, when there are @EventHandler annotated methods, the EventService is
treated as a dependency. So it is no longer necessary to explicitly list
EventService.class before other services when constructing a Context
with an explicit list of service interfaces.
Add unit tests for UsageService behavior