This introduces a registry_populating signal, which allows code to

lazily register entries in the registry during its population phase.

This prevents an application from having to register custom entries at

some point in its initialization process, or to try to guess a good time

to do so before the registry might be used. Applications can simply

listen to this signal and register when fired.
The signal provides the usual sender argument, which is the class of

the registry, and a registry argument for the instance. To be truly

effective, subclasses of Registry, EntryPointRegistry, etc. should

be used, so that the sender can always point to something specific.

Djblets and Review Board unit tests pass.
Tested with some new code in Review Board that uses this signal instead

of populating on initialization. Confirmed that the signal fired at the

appropriate time and the new information was registered.