Add read/write support for content controls (structured document tags / SDT). Upstream python-openxml#155 (15 comments), python-openxml#224 (15 comments).

API Design

Read:

• document.content_controls — iterate all SDTs in the document body
• paragraph.content_controls — SDTs within a paragraph
• ContentControl: .tag, .title, .type (plainText, richText, checkbox, comboBox, dropDown, date, picture)
• ContentControl.text — get/set the current value
• ContentControl.checked — for checkbox type

Write:

• paragraph.add_content_control(type, tag, title) — add an inline SDT
• body.add_content_control(type, tag, title) — add a block-level SDT

XML

• Block-level: w:sdt > w:sdtPr (properties) + w:sdtContent (content paragraphs)
• Inline: w:sdt within w:p > w:sdtPr + w:sdtContent (runs)
• w:sdtPr children: w:tag, w:alias (title), w:lock, w14:checkbox, w:comboBox, w:dropDownList, w:date

Upstream issues: python-openxml#155, python-openxml#224