ticket 1669 - improve i18n docstring extraction

This patch reverts the use of pygettext for i18n string extraction. It
was originally introduced because the help documentation for commands
are in the class docstring and module docstring.

Docstrings are a Python construct whereby any string which immediately
follows a class declaration, function/method declaration or appears
first in a module is taken to be the documentation for that
object. Python automatically assigns that string to the __doc__
variable associated with the object. Explicitly assigning to the
__doc__ variable is equivalent and permitted.

We mark strings in the source for i18n translation by embedding them
in _() or ngettext(). Specialized extraction tools (e.g. xgettext)
scan the source code looking for strings with those markers and
extracts the string for inclusion in a translation catalog.

It was mistakingly assumed one could not mark for translation Python
docstrings. Since some docstrings are vital for our command help
system some method had to be devised to extract docstrings for the
translation catalog. pygettext has the ability to locate and extract
docstrings and it was introduced to acquire the documentation for our
commands located in module and class docstrings.

However pygettext was too large a hammer for this task, it lacked any
fined grained ability to extract only the docstrings we were
interested in. In practice it extracted EVERY docstring in each file
it was presented with. This caused a large number strings to be
extracted for translation which had no reason to be translated, the
string might have been internal code documentation never meant to be
seen by users. Often the superfluous docstrings were long, complex and
likely difficult to translate. This placed an unnecessary burden on
our volunteer translators.

Instead what is needed is some method to extract only those strings
intended for translation. We already have such a mechanism and it is
already widely used, namely wrapping strings intended for translation
in calls to _() or _negettext(), i.e. marking a string for i18n
translation. Thus the solution to the docstring translation problem is
to mark the docstrings exactly as we have been doing, it only requires
that instead of a bare Python docstring we instead assign the marked
string to the __doc__ variable. Using the hypothetical class foo as
an example.

class foo(Command):
    '''
    The foo command takes out the garbage.
    '''

Would become:

class foo(Command):
    __doc__ = _('The foo command takes out the garbage.')

But which docstrings need to be marked for translation? The makeapi
tool knows how to iterate over every command in our public API. It was
extended to validate every command's documentation and report if any
documentation is missing or not marked for translation. That
information was then used to identify each docstring in the code which
needed to be transformed.

In summary what this patch does is:

* Remove the use of pygettext (modification to install/po/Makefile.in)

* Replace every docstring with an explicit assignment to __doc__ where
  the rhs of the assignment is an i18n marking function.

* Single line docstrings appearing in multi-line string literals
  (e.g. ''' or """) were replaced with single line string literals
  because the multi-line literals were introducing unnecessary
  whitespace and newlines in the string extracted for translation. For
  example:

  '''
  The foo command takes out the garbage.
  '''

  Would appear in the translation catalog as:

"\n
  The foo command takes out the garbage.\n
  "

  The superfluous whitespace and newlines are confusing to translators
  and requires us to strip leading and trailing whitespace from the
  translation at run time.

* Import statements were moved from below the docstring to above
  it. This was necessary because the i18n markers are imported
  functions and must be available before the the doc is
  parsed. Technically only the import of the i18n markers had to
  appear before the doc but stylistically it's better to keep all the
  imports together.

* It was observed during the docstring editing process that the
  command documentation was inconsistent with respect to the use of
  periods to terminate a sentence. Some doc had a trailing period,
  others didn't. Consistency was enforced by adding a period to end of
  every docstring if one was missing.

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