Ftw-builder

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- DxBuilder: Fix check if field is present (to determine if default values should be set).
[lgraf]

- DxBuilder: Make sure default values are set before adding content to container.
[lgraf]

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- Update object_provides in catalog when using provides().
[jone]

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- Provide a real image (1x1 px GIF) for the Archetypes ImageBuilder.
[mathias.leimgruber]

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- Implement ATImage builder.
[jone]

- Reindex object security after setting local roles for a principal.
[mathias.leimgruber]

- Support "on" keyword argument for with_roles method in group builder.
[mathias.leimgruber]

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- Add `providing()` method to Plone builder,
letting the object provide interfaces.
[jone]

- Don't use IDNormalizer for Mail. It handles Umlauts weird.
[tschanzt]

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- Add groups builder.
[jone]

- Add users builder.
[jone]

- Added modification date setter for PloneObject Builders.
[phgross]