Collective-messagesviewlet

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

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

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

- 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]

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

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

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

- Increased coverage by using vocabulary methods instead of redefining it in tests.
[gbastien]
- Added new example message: bad browser (not Firefox) warning
[sgeulette]
- Use plone.formwidget.datetime to have hour at 0 by default.
[sgeulette]
- CSS fix : display 'cursor: pointer;' when hovering the close button.
[gbastien]
- Added 'MessagesConfig' to site_properties.types_not_searched.
[gbastien]

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

- Added parameter 'activate' to utils.add_message that makes it possible to create
an 'activated' message directly.
[gbastien]
- Adapted tests to use utils.add_message instead of duplicating this code.
[gbastien]

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

- Do not pass a default 'context' in utils._ to avoid strange ConnectionStateError.
[gbastien]
- Give context to translate method.
[sgeulette]
- Corrected and added icon type images.
[sgeulette]