Plone.app.viewletmanager

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Fixes:

- Use zope.interface decorator.
[gforcada]

- Correctly log exception if viewlet rendering failed.
[jensens]

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- Fix manage-viewlets for Plone 5
[pbauer]

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- Rename ``_uncatched_errors`` to ``_exceptions_handled_elsewhere``
[jean]

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- flake8 fixes and general cleanup.
[gforcada]

- Sort skins and viewletmanagers on exports to create stable exports.
This fixes: https://github.com/plone/plone.app.viewletmanager/issues/7
[gforcada]

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- Use the ``!important`` directive for the ``.hide`` and ``.show`` CSS
declarations on the ``manage-viewlets`` view. Twitter Bootstrap is using
``!important`` on these class names too, which made the viewlet management
view unusable.
[thet]

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- Do not catch conflict errors and keywordinterrupt in viewlet manager.
We can programmatically setup the exceptions that are not caught.
[thomasdesvenain]

- If render fails, be more verbose about the exception to know where and how it
happens in the stack. [kiorky]

- refactor JS in template.
[petschki]