Expose an `application.addCss` method and hide `application.cssSelectors`
from end-user typings. Update StyleScope's with new application selectors
on style application.
NOT updating existing controls/views unless reloaded, or forced (via a
change to className, etc).
- Use relative imports in place of most of our absolute ones.
- Add "private" ambient modules for modules that we need to import using
an absolute path (e.g. when app/.../test-something.ts needs to import
ui/styling/style-scope)
