Collective.task

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

- Added validator on assigned_user
[sgeulette]
- Corrected table column style.
[sgeulette]
- pep8 on imports
[sgeulette]

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

- Modified enquirer field to be LocalRoleField. Use overridable vocabulary.
[sgeulette]
- Added parents_assigned_groups and parents_enquirers field to manage parents rights
[sgeulette]
- Block local roles on task content
[sgeulette]
- Added special index value for empty field.
[sgeulette]
- Use Owner role in workflow
[sgeulette]

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

- Set initial_trigger to true.
[sgeulette]

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

- Add a get_full_tree_title method returning the path title of a task until its parent container.
[fngaha]

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

- Adapted ITask to add method getting the highest parent.
[sgeulette]
- Colorize states
[sgeulette]
- Add icons for transitions
[sgeulette]
- Add viewlet displaying task parents
[sgeulette]

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

- Modify a copy of the field.
[sgeulette]