Continue numbering of list by identifier

This implements the text:continue-list attribute which specifies the list
whose numbering should be continued with a unique identifier.

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1418832_253892949
Moved generation of list level CSS selector

The code for generating the appropriate CSS selector based on the level
the list style applied was being duplicated so I have pulled it out into a
separate function.
Remove dead code from OdfCanvas

The removed code was intended to implement continue list functionality.
However this code no longer worked due to a regression introduced in commit 3ae9e0c.

As I have refactored this area and implemented the continue list
functionality in the new list styles class this old code is no longer
required.