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.
Add tests for a built-in extension type to framework to allow app.run
to be invoked via bundle extensions. This supports stylesheet
injection, which should happen on load but isn't triggered
from any particular part of the page. WTD-591.
