Imio.history

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- Remove revision columns when unused.
[DieKatze]
- In ImioRevisionHistoryAdapter.getHistory, take into account the
'checkMayView' parameter by implementing a 'mayViewRevision' method so it
is possible to restrict access to a specific revision if necessary
[gbastien]

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- Added history-version-preview view that is called by default in the
content_history but that renders nothing. It is made to be registered for a
relevant content_type so it is possible to display a preview of a versioned
object directly in the history popup
[gbastien]

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- Add revision history management.
[cedricmessiant]

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- Added helper method 'utils.getPreviousEvent' that will receive an event
as parameter and will return the previous event in the workflow_history
if found
[gbastien]

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- Do not reverse workflow_history in ImioHistoryAdapter.getHistory
as it is for display purpose, do this in the IHContentHistoryView.getHistory
[gbastien]
- Added parameter 'checkMayView' to ImioHistoryAdapter.getHistory to be able
to enable/disable mayViewComment check while getting the workflow_history
[gbastien]

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- Be defensive in getHistory, do not fail if no workflow used or
if element has no workflow_history attribute
[gbastien]