Plone.app.folder

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- Add `IATBTreeFolder` to `implements` list of `ATFolder` replacement.
[thet]

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- Copy the `index_html` method from `ATContentTypes` to better support WebDAV.
[davisagli]

- Add in-place migration code.
[witsch]

- Work around imports no longer present in Plone 4.0.
[witsch]

- Briefly document the `plone.app.folder.tests.bbb` usage.
[wichert]

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- Initial release as factored out from `plone.folder`.
[witsch]