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 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.
Added tests for continue lists and multi level lists

Added some tests that look at the computed style to verify the correct
rules have been applied by ListStylesToCss for these list types.

The behaviour of getComputedStyles differs between browsers so some hacks
have been used in the tests.