libmsooxml: processing of lists - cleaning and update

* Improved readability and cleaning of the text:list related parts.

* Improved the auto-numbering logic to restart and continue the numbering of list items.
Started to use the default value for the startAt attribute of the buAutoNum element.

* Do not create a new list when the list style did not change, continue the current list.

* Disabled the simplified lists processing approach for docx and xlsx. Let's use the same
approach as for pptx, there should not be any differences except for the list-style
definition.

* Check if a style should be inserted into styles.xml also for docx. When does a docx
document uses the DrawingML format? I don't have any test files. No regressions were
found on docx documents we have for cstester. If DrawingML is used for a text-box,
then we need a specific logic to create the list style for a list inside of a text-box.