Imio.prettylink

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- Use `self.context.wl_isLocked()` to show locking icon so it does not break
on non lockable objects.
[gbastien]
- Added boolean `display_tag_title` parameter.
[sgeulette]

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

- Handle icon path correctly.
[sgeulette]

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- Do simplify link to file the download by just appending the download to
the URL, this is enough if current context has a primary field.
[gbastien]

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- Take `ACTUAL_URL` stored in the REQUEST into account in the `getLink` caching
cachekey to manage the fact that the URL to the element changed. This can
be the case when accessing element thru different domains or if a parent
of the element was renamed.
[gbastien]

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- Do not break in `PrettyLinkAdapter.getLink_cachekey` if context does not have
a workflow.
[gbastien]

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

- Added caching for `PrettyLinkAdapter.getLink`, the cachekey returns context,
modified, is_locked, review_state and every parameters defined in
`PrettyLinkAdapter.__init__`.
[gbastien]
- Moved link computation from `PrettyLinkAdapter.getLink` that is now a cached
method to `PrettyLinkAdapter._getLink` so it is possible to call it directly
without caching or to override it.
[gbastien]
- Finalized testing configuration to be able to test the getLink caching.
[gbastien]