ODPresentation Writer : a placeholder is written as an ordinary frame, so a title stops being a title

[dkulyk]

Nothing in Writer/ODPresentation emits presentation:class. A shape marked

$shape->setPlaceHolder(new Placeholder(Placeholder::PH_TYPE_TITLE));

is written as an ordinary draw:frame, so the title of a slide stops being a title. Measured on one
report written both ways and converted by LibreOffice: the PPTX carries 6 /H elements in the
tagged PDF, the ODP none. The ODF LibreOffice itself makes of that deck holds
presentation:class="title" six times, so the format is not the limitation.

The class alone buys nothing

Measured on a two-shape deck -- one title and one body placeholder -- written by this library and
converted to a tagged PDF:

the frame carries	ODF 1.2	tag tree
what the Writer emits today	valid	Div:2, P:2
+ presentation:class	valid	Div:2, P:2 -- unchanged
+ presentation:class + draw:layer="layout"	valid	Div:2, P:2 -- unchanged
presentation:style-name in place of draw:style-name, no class	valid	Div:2, P:2 -- unchanged
the class and presentation:style-name in place of draw:style-name	valid	Div:1, H:1, P:2
the same deck written as PPTX, for reference	--	Div:1, H:1, P:2
the ODP LibreOffice makes of that PPTX	--	Div:1, H:1, P:2

A title becomes a title only when the class is paired with a style of family presentation. Either
half on its own leaves the frame a paragraph.

The two style attributes are mutually exclusive

ODF 1.2 spells them as a <choice> (OpenDocument-v1.2-os-schema.rng:6412-6437):

<choice>
  <group> draw:style-name?         draw:class-names?         </group>
  <group> presentation:style-name? presentation:class-names? </group>
</choice>

A frame carries one group or the other; writing both answers with
Invalid attribute style-name for element frame. So writeTxtStyle() (Content.php:971) has to
emit pr{shapeId} of family presentation instead of gr{shapeId} of family graphic for a
placeholder, carrying the same style:graphic-properties. A replacement, not an addition.

style:parent-style-name="Default-title" is not needed: LibreOffice writes one, but measured
without it the file is valid and the /H still appears. Master pages and presentation-page-layout
stay out of this.

The map

The branch already exists -- getPlaceholderField() (Content.php:717) reads the placeholder for
text:page-number and text:date. What is added beside it is the class map, and ODF does not spell
three of the seven the way OOXML does:

Placeholder::PH_TYPE_*	presentation:class
title	title
body	outline
subTitle	subtitle
ftr	footer
dt	date-time
sldNum	page-number
chart	chart

presentation-classes in the schema (:7752-7770) is a closed list of seventeen values, so a typo
fails assertIsSchemaODF12Valid() rather than shipping.

PR to follow.