Zope.app.container

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- Fixed tests using a newer zope.publisher that requires zope.login.

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- Fixed test_directive. Some parts of zope.app.publisher were moved
to zope.browsermenu and zope.browserpage.

- Moved tests/test_view_permissions.py to browser/tests.

- Added undeclared install dependency on ``zope.app.publisher``.

- Test no longer use deprecated ``zope.testing.doctestunit`` but
python's ``doctest`` instead.

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- Moved ``IAdding`` interface to ``zope.browser.interfaces``, leaving
BBB imports.

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- Show a "nothing to add" message instead of empty list in the
adding view, if there's nothing to add.

- Don't show the "Add" menu item if there's nothing to add.

- Adapt to the removal of deprecated interfaces from
``zope.component.interfaces``. Now ``IAdding`` inherits from
``zope.publisher.interfaces.browser.IBrowserView``.

-------------------

- Updated test to accomodate "Pythonic" exception now raised from
``__setitem__`` provided by ``zope.container`` (``KeyError`` instead
of ``zope.exceptions.UserError``).

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- Remove long-time deprecated ``IContentContainer`` class.

- We now rely on a new package called ``zope.container``, which
contains the basic implementation of ``zope.container`` and is
intended to have less dependencies. We have gone through a wide
range of packages and updated their dependencies to point to
``zope.container`` so that they will also have less indirect
dependencies.

For backwards compatibility we have left the original modules in
``zope.app.container`` in place and have placed imports to make sure
the symbols exist in their original locations.