Provide each text:list with a unique CSS counter

Giving each list a unique counter allows us to implement continued
numbering functionality while avoiding unexpected numbering due to how CSS
counter values are scoped to an elements sibilings or children.
Implement continued numbering

This adds support for the text:continue-numbering and text:continue-list
attributes. The text:continue-numbering attribute specifies whether to
continue the immediately preceding list if it has the same style.

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#attribute-text_continue-numbering

The text:continue-list attribute specifes the unique identifier of a list
to continue numbering from in the document. These must also have the same
style.

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#attribute-text_continue-list

https://tools.oasis-open.org/issues/browse/OFFICE-3558
Remove dead code from OdfCanvas and OdfUtils

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.

The removed code in OdfCanvas also made use of the
getFirstNonWhitespaceChild function in OdfUtils and the removed code was
the last remaining usage of it. As it is preferable to make use of DOM API
calls which give us the desired behaviour such as
element.firstElementChild I have removed this function.