Add priority ordering to loaded extensions in each category;
this allows control over the resulting order of extensions
acquired and used within the application. WTD-590
Bring in changes from 'sandbox' branch. These include:
* Reconfiguring require's baseUrl such that relative
paths work as expected in define call dependencies.
Previously this only worked as expected in the
framework bundle, since data-main points there.
* Add support for a 'constants' category of extension,
to define application constants from bundles. This
allows services to treat static values (such as
persistence URIs) as injectable dependencies.
* Various assurances that properties from extension
definitions will be exposed upon resolved
implementations, even after partial construction.
WTD-572.
Satisfy dependencies on empty extension sets when
necessary; extensions which depend on extension
categories which do not exist then get an empty
array. WTD-518.
