deform2 branch: put drag icon in panel header when draggables are mappings and/or sequences

[mcdonc]

When draggables of an orderable sequence are themselves mappings or sequences, the panel header should be a drag target (it is, now, in the case of mappings but not sequences).

These panel headers should also probably show a little up-down arrow in them to indicate they are themselves draggable.

[stevepiercy]

Duplicate https://deformdemo.pylonsproject.org/sequence_of_sequences/ and add orderable=True in both levels of SequenceWidget() for the functional test. Add Name and Title, then Add Names and Titles, and finally Add Name and Title in the last Add Names and Titles to play around with orderables.