This update adds a new user interface library foundation and supporting tools that make Odoo’s web screens easier to build, test, and maintain. It also introduces reusable dialog and file upload components while removing obsolete mail announcement code, helping future interface improvements ship more consistently.

This PR has several purposes:
 - add the owl library to Odoo;
 - defines two JavaScript modules web.env and web.test_env that should make
   easier to work with and test owl components.