ObjectIndex: eliminate long lines
ObjectIndex: eliminate warnings
Remove duplicate blank lines
ObjectIndex: add missing section header
ObjectIndex: allow lazy registration of objects
AbstractSingletonService: register instances lazily

To be more precise, the objects were previously registered lazily upon
the first call to SingletonService#getInstances(). Unfortunately,
calling ObjectService#get or ObjectService#getAll before that failed to
return the singleton instances, because they had not yet even been
instantiated. To address this problem, we now register them using the
ObjectIndex's newly minted addLater feature, which lets us register a
callback to create objects only when they are needed.
Merge branch 'lazy-objects'

This branch adds a feature to the ObjectIndex to register a callback for
later addition of objects which do not yet exist. It facilitates lazy
initialization of services by giving them a mechanism to tell the object
service: "Ask me about my objects later, when you need to know."

With this change, we can eliminate several hacky bits of code in ImageJ2
and KNIME, working around the fact that objects generated by lazily
initialized services were inconspicuously missing from the object
service prior to those services' actual initializations upon first use.