Collective.messagesviewlet

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- Corrected import step dependencies to avoid unresolved warning.
[sgeulette]

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- Generate new uid when message is activated or re-activated.
[sgeulette]

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- Add <span> around cross <img> to ease override.
[mgennart]

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- Corrected image path.
[sgeulette]
- Decrease space between messages
[sgeulette]

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- Added default message to warn that application only runs correctly on Firefox
and Chrome, in addition to the already existing message that warned about the
application only running correctly on Firefox.
[gbastien]
- Use CheckBoxWidget for IMessage.required_roles` to ease selection when
displaying several elements.
[gbastien]
- Use RadioFieldWidget for Bool fields `IMessage.can_hide` and
`IMessage.use_local_roles` so it is displayed correctly on the view,
especially when it is False.
[gbastien]

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- Use INavigationRoot instead of IPloneSiteRoot to check if context is homepage.
[bsuttor]