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- (broken) rendering of field lists
- ignore substitution definitions
LabeledFlowable: a base class for ListItem

This class can also serve as the base class for footnotes.
Render label and content each in their own container

- fully specify list style in the style sheet
- specify 'label_ width' and 'label_spacing' for LabeledFlowable
Footnote improvements

- make use of LabeledFlowable
- simplify LabeledFlowable/ListItem and get rid of ListItemNumber
- fix footnote handling in reStructuredText
- suppress inline rendering of the footnote
- find the footnote corresponding to the footnote_reference
- give the footnote classes more correct names