Implement 'runs' as a built-in category of extension, to support
bundle addition of functions to run when the application first
starts. Specifically supports loading of stylesheets, WTD-591.
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.
