oxml: add CT_Document type
accept: add paragraph_style.feature
doc: document CT_PPr analysis
text: add Paragraph.style getter
text: add Paragraph.style setter
oxml: add CT_PPr.style getter
oxml: add CT_P.style getter
oxml: add CT_PPr.style setter
oxml: add CT_PPr.new()
oxml: add CT_P.style setter
tmpl: add default styles to default.docx
body: add _Body.clear_content()
oxml: add CT_Body.clear_content()
doc: document CT_Styles analysis