Implement viewlet dummy ordering needed for Python 3 sorting

[ale-rt]

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[ale-rt]

The reason for this PR is that viewlets are sorted here:

• https://github.com/plone/plone.app.viewletmanager/blob/master/plone/app/viewletmanager/manager.py#L84

The line brings my name, but that sorting is in place since 11 years and looks like:

# then sort the remaining ones
# Copied from Five
remaining = sorted(name_map.items(), lambda x, y: cmp(aq_base(x[1]), aq_base(y[1])))

x[1] and y[1] are viewlets instances.

It seems to me that there isn't actually a good criterion that can be used to sort that remaining viewlets (the ones that are not sorted by the IViewletSettingsStorage utility).

I think we should remove the sorted call in https://github.com/plone/plone.app.viewletmanager/blob/master/plone/app/viewletmanager/manager.py#L84
As an alternative we may provide an attribute that can be used for sorting them (order? with a default of 0?) and the __lt__ operator should check it.