Sub/superscript in reStructuredText
Determining the width of a rendered flowable

- make Flowable.render() also return the rendered width
- make Flowable.flow() return the width instead of the (unused) height
Allow stretching the label width based on the actual label

- specify minimum and maximum label width

To do:
- don't store the calculated label width in the LabeledFlowables object
- the label style attributes should be moved to in LabeledFlowables
- add option to render the labeled flowable below the label