Add uninstall profile.

:construction: the tests are currently failing, i need input on how to uninstall as "clean" as possible:

There are two profiles to uninstall. The ``extjs`` profile disables an extjs css file and adds overrides in ftw.table's own extjs css file. How should i uninstall this in a clean way? The two options i see:
- Add a setuphandler that conditionally resets the extjs configuration when uninstalling, but only when extjs is enabled/installed
- Don't do anything, warn the user in the README and add set ``cssregistry.xml`` to be skipped for the extjs profile uninstallation test

@jone do you see another way, and if not which of the above options would you prefer?