Grokcore.site

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- Python 3 compatibility.

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- When removing a site make sure reference are removed from the parent
site.

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- Update tests.

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- Add a new directive ``install_on`` that is usable on a site. This
directive let you customize when (namely the event) to install the
configured local sites.

- Rename ``ApplicationInitializedEvent`` to ``ApplicationAddedEvent``.

- When the ``IApplicationAddedEvent`` is triggered the new application
will be current Zope local site. The site is restored after the
event.

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- Exposed ApplicationInitializedEvent and IApplicationInitializedEvent.

- Added the missing import for the exposed IApplication interface.

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- Moved the directive `site` from Grok to this package.

- Moved the component `Application` and all the related utilities from Grok
to this package.