acpt: add font.italic feature
test: update test_text.py to current standards
test: first cut declarative XmlBuilder
text: refactor Describe_Run to pytest
text: refactor Describe_Paragraph to pytest
test: add with_child() capability to XmlBuilder
text: reorder methods in _Paragraph

Also converted alignment and level properties to use decorators rather
than property built-in. Reordered tests to match method order.
text: refactor add_run() tests

Along the way:

* renamed _Paragraph.__p to _p
* renamed _Run.__r to _r
oxml: add CT_TextParagraph.add_r()

Along the way:

* migrate custom element class registration to oxml.__init__.py
* refactor _Paragraph.add_run() to use add_r()
* move actual_xml() to tests.unitutil
* update CT_TextCharacterPropertiesBuilder for double-duty with <a:rPr>
and <a:endParaRPr>
oxml: add CT_TextParagraphProperties
text: add _Paragraph._pPr property
oxml: add CT_TextParagraph.get_or_add_pPr()
oxml: add CT_TextParaProps.algn

Also:

* refactored _Paragraph.alignment to use .algn
* removed CT_TextParagraph.get_algn()
* reordered Describe_Paragraph fixtures in test_text.py
oxml: add CT_TextParaProps._set_attr()

as means of implementing 'algn' property setter
oxml: add CT_TextParagraph.remove_r_child_elms()
text: refactor _Paragraph.clear() to use oxml calls
oxml: add CT_TextCharacterProperties
oxml: add CT_TextParaProps.get_or_add_defRPr()
text: refactor _Paragraph.font to use ._defRPr
oxml: add CT_RegularTextRun