Docutils: Documentation Utilities Code

One more Document Tree documentation review.

Rephrase and reformat for more brevity and clarity.
Fix typos, add crosslinks.

Less gaudy colors in the element hierarchy diagram.

Make the "Element Reference" introduction a sub-section,
so it turns up in the local ToC.

Update analogies to HTML (now referring to HTML5 rsp. the "living standard").
Fix analogy to `<h1>` to `<h6>`: these mark titles (headings), not sections.

Reduce redundancy: Only list including "Parameter Entities" if not
implied by the element category).

Update description of `<line-block>` (it used to be a
sort of literal-block but is is now rather a plain list,
cf. docs/dev/rst/alternatives.html#syntax-for-line-blocks).

Small section for "placeholder" parameter entities.

Move the description of "Pseudo-XML" and the "Names and identifiers"
section content to a new "Glossary" section.

Bibliography entry for docutils.dtd.

Small fix for target-highlight in "responsive" stylesheet (HTML5).

Highlight complete citation, not only the label.

Wrap setting the default "locale" in try/except, report locale.Error as warning.

Docutils command line tools use `locale.setlocale()` to set the locale to the user's
default locale. This may lead to a `locale.Error` if there are problems in the
systems locale setup. As locale settings are not critical for the working of
Docutils (but may affect the output in certain instances), we now clear the
exception and report a WARNING.

Fixes bug #495.

rename .txt to .rst (left over)

rename .txt to .rst (left over)

move .txt to .rst (left over)

Fix recursion in `nodes.Element.get_language_code()`.

Because of a typo, `Element.get_language_code()` did not find language
tags in parent elements but just return the fallback value.

Thanks to Marcello Perathoner for the report.
http://permalink.gmane.org/gmane.text.docutils.devel/10544

Condense "details" lists in Document Tree documentation.

More concise style in the sections listing doctree element details.
(Reduce boilerplate text and repetitions,
use phrases instead of full sentences if applicable.)

Doctree documentation revision.

Update documentation regarding the "colwidth" attribute handling.
and the included `Exchange Table Model`.

Unify name use.

Revise/fix hyperlinks.

Improve wording/formatting.

Correct description of the "names" doctree element attribute:

Under specific circumstances, an element may have both,
``names`` and ``dupnames`` attributes.
This is intentional to provide a means to solve conflicts in
implicit hyperlink targets (e.g. two section headings with similar title),
e.g. ::

My Title
--------

.. _my title:

my title
--------

`My title`_ links to the second section.

Start reconciling "colwidth" attribute definition and use.

First, backwards compatible changes get the handling of the
`<colspec>` element's "colwidth" attribute in line with the
definition in the Exchange Table Model.

The "colwidth" attribute is defined in the "Exchange Table Model"
as either a *proportional measure* or a *fixed measure*.
Currently, Docutils supports only unitless values and interpets them
as proportional measures while the Exchange Table Model interprets
unitless values as fixed measures with default unit “pt”!

The new optional argument "unit_pattern" for `nodes.parse_measure()`
allows configuring what is recognized as a unit.

The new validation function `nodes.validate_proportional()`
for the "colwidth" attribute also accepts conformant values like
"5.2*" (for 5.2 times the "unit proportion").
For backwards compatibility reasons, the attribute is still stored
as numerical value in Python doctree element instances.

New auxiliary method `nodes.colspec.propwith()`:
returns the "colwidth" attribute as number if it is a
proportional measure.

Announce future changes in the RELEASE_NOTES

Add/modify test cases.