Fix multi level list numbering

Changed the CSS counter used to the nested counter to give us the correct
numbering behaviour in multi level lists.
Removed fixed width sizing of list labels

Replaced the fixed width sizing which was used to provide spacing after
the list label with a constant padding value instead. The fixed width sizing caused large
list labels to wrap to the next line which was undesireable.

The padding value has been chosen so that lists render ok until the list
label size functionality is implemented as per the ODF spec.

The behaviour we should be following is detailed here:
http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#attribute-text_label-followed-by

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#element-style_list-level-label-alignment