Use SciJava's @Parameter instead of @ModuleInput
AbstractTreeSource: move to main source package
Rename Module to Op

It may not be the *best* name, but it does avoid a name clash with
ImageJ's Module idea (even if the two things are a bit similar).

Op is an OK working name for the time being, at least.
Merge TreeSource event API into Op itself

This means that all Ops now publish events when they are updated,
rather than only TreeSources.
Rename isEquivalentModule to isEquivalentOp
Op: extend SciJava RichPlugin

This paves the way for discovery of available Ops using the SciJava
plugin discovery mechanism.
imglib2-descriptors: enable SezPoz annotation processing
DescriptorTest: remove unnecessary import
Add @Plugin annotations for all concrete ops!
Add first cut at an OpService to manage Op plugins
Add more granular subtype descriptor interfaces

This granularity will allow those types to be specifically queried from
the plugin service.
Op: remove obsolete priority() method

Instead, we use the SciJava Prioritized API.
DescriptorRepository: lean on the OpService

We can now identify compatible Ops using the OpService instead of the
old method.