Docutils: Documentation Utilities Code

Document two more doctree elements + other documentation updates.

Document `<problematic>` and `<system_message>` doctree elements.

Various more or less related documentation corrections and updates.

Support CSS3 `length units`_.

Add "ch", "rem", "vw", "vh", "vmin", "vmax", and "Q" to the length units
supported by reStructuredText.

Note that some output formats don't support all units.

HTML: CSS3 units are supported by all contemporary browsers.

XML, pseudoxml: Docutils Document Tree elements validate
"any run of letters or a percent sign" as a measure's unit.

LaTeX: the writer emulates non-native units with custom length macros.

ODT: the writer only supports fixed length units (by converting to "cm").
Support for the unit "Q" is added.

man: the writer ignores length specifications.

Fixes feature-request #57.

LaTeX writers update.

Deprecate `writers.latex2e.SortableDict`.

Remove `writers.latex2e.Table.get_caption` (obsolete).

Test and fix local ToC without global ToC.
Package "minitoc" needs a ``\faketableofcontents`` in this case so that a
".toc" file is generated.
Move the conditional insertion code in `depart_document()` before the
conversion of `self.requirements` from a dict to a list, so that the
test works again as intended.

Minor documentation fixes.

HTML5: Fix display of inline images in section headings.

Add selectors for an image inside h1 ... h6 to the rule for inline images.

Update/fix "standard" functional test sample.

Fix a factual error
(footnotes must be indented at least 1 space, not 3).

Reword HTML-centric descriptions -- we use the sample for several
writer tests.

Revert change of expected pseudo-xml sample extension from "txt" to "rst"
(side-effect of the switch to "rst" extension for rst sources).

Revert [r9902] (right align code line numbers with CSS).

The change in [r9902] has the side-effect of a spurious space
before the first line of a code block without line nubers.

Right-aligning via CSS is not required, as line numbers are already
padded by the parser.

Revert.

LaTeX writer: stop requiring "ifthen.sty", fix Babel warning.

Replace use of ``\ifthenelse`` from "ifthen.sty" with the
eTeX primitive ``\ifdefined``.

Mark the main language when loading "babel" to prevent a warning
about another language being processed last. This happens when there
is more than one language in the document as we put the main language
also in the document options (for other packages to see).

Use `nodes.parse_measure()` in rST directive option conversion.

Simplify `parsers.rst.directives.get_measure()` by using
`nodes.parse_measure()`.

Sort the list of supported length units in the order used in the
rST specification (based on the CSS specification).

Update and extend the lenght value handling tests.
Clarify and test behaviour:

* case of length units is preserved (in contrast to CSS)
* the restriction of supported units is a rST feature
while the Docutils Doctree allows any run of ASCII letters + µ
or a percent sign as units on attributes with value type "measure".

Update documentation around length specifications.