Make numbering definitions writable - define a list from scratch #98

#73 gave us a good read side for numbering — Paragraph.numbering, Paragraph.list_number, Document.list_numbers, NumberingLevel.number_format, .level_text, .start, format_number() — plus set_numbering(), remove_numbering() and restart_numbering() on the write side.

What is missing is the ability to define a list. NumberingLevel is read-only: start, num_fmt, lvl_text, lvl_restart, is_lgl and p_style on CT_Lvl are all getters with no setters, and Numbering has no add_definition(). CT_Numbering.add_num() exists but only takes an abstractNum_id that must already be there.

So you can apply a list definition the document already has, and you can restart one, but you cannot create a numbered list with a format the template does not already contain — no 1) when the template only has 1., no custom bullet character, no per-level indent. In practice people fall back to hand-building w:abstractNum XML and appending it, which is exactly the kind of thing the library should be doing for them.

Where the prior art is

jlovegren0/python-docx, branch master (and footnotes), is the most complete numbering-definition layer of any of the forks. It adds constructors and writable attributes for CT_AbstractNum, CT_Lvl, CT_NumFmt, CT_LevelText, CT_LevelSuffix, CT_MultiLevelType, CT_LvlLegacy and CT_NumPicBullet, along with the ST_NumberFormat, ST_LevelSuffix and ST_MultiLevelType simple types in docx/oxml/simpletypes.py and an abstractNum_having_abstractNumId() lookup.

Our CT_AbstractNum and CT_Lvl already exist with the read accessors and lvl_having_ilvl(); WD_NUMBER_FORMAT already covers ST_NumberFormat. This is filling in the write side of classes we have, not new ones.

Yiqing-Zhou/python-docx and revvsales/python-docx-1 (Document.get_new_list(abstractNumId)) both do smaller versions of the same thing.

Shape

• Setters on CT_Lvl for start, numFmt, lvlText, lvlRestart, suff, lvlJc and the level w:pPr indent
• w:suff (ST_LevelSuffix: tab, space, nothing) is not modelled at all — it is what controls the gap between the number and the text, and is a common ask
• Numbering.add_definition(levels) -> NumberingDefinition, allocating a free abstractNumId and numId and wiring the w:num -> w:abstractNum link
• A NumberingLevel write side matching the existing read properties
• Probably a helper for the two common cases, a bulleted list and a decimal-with-sublevels list, since assembling nine levels by hand is the tedious part

Watch out for

• CT_Numbering is an xsd:sequence — all w:abstractNum elements precede all w:num. _tag_seq already reflects this; new definitions must be placed accordingly and the placement asserted in a test.
• Word rejects a w:abstractNum whose w:lvl elements are not in ascending w:ilvl order.
• w:nsid and w:tmpl are what Word uses to recognise a list definition as one of its own. Check ref/xsd/wml.xsd and decide whether to emit them; documents open fine without them but the list may not round-trip through Word's list gallery.