...mainly to test how well/poorly extension help is supported by
--with-help=html, but also as a kind of reference implementation.  (There are
also extensions in odk/examples/ that contain help content, namely
odk/examples/DevelopersGuide/Extensions/DialogWithHelp/ containing context-
sensitive help for a dialog, a topic that is not covered here.)

See <https://wiki.openoffice.org/wiki/Documentation/DevGuide/Extensions/
Help_Content#Integration_into_the_OpenOffice.org_help_system> for the ways an
extension's help content can be integrated into the "classic" (plain
--with-help) help system (citing sub-section headings used in that document):

* Help Viewer Contents page:

  The contained help.tree provides a "The test-passive Extension" section with
  a "The test-passive Extension" page (main.xhp).

* Help Viewer Index page:

  The contained main.xhp provides index entries for "test-passive extension" and
  "extensions"-"test-passive", and---only on the index pages of Calc and Writer
  ---for "test-passive extension in Calc and Writer".

* Help Viewer Find page:

  The contained main.xhp is indexer="include", so it should be found by e.g.
  searching for "bla bla".

* Context sensitive help and extended tool tips:

  The contained Addons.xcu has been extended with a small (floating by default)
  toolbar containing a "native" button (doing the same as the "passive - native"
  menu entry), and the contained main.xhp provides an extended tooltip of "Show
  the test-passive extension's native dialog" for that toolbar button (displayed
  when using "Help - What's This?" and hovering over the button).

  For context-sensitive help for dialogs, see the mention of
  odk/examples/DevelopersGuide/Extensions/DialogWithHelp/ above.

The contained help is only provided in "en" localization, which should thus be
used as fallback regardless for which locale LO is run.

The help-related XML files need to provide identifiers in lots of places, many
of which appear to be unused for the given example uses.  I specified those as
empty strings.

The workdir/Extension/test-passive.oxt extension can be built with `make
Extension_test-passive`.

Change-Id: I0e75f35df85683c9fae1d1384fa6afeaeb8a687d
Reviewed-on: https://gerrit.libreoffice.org/55736Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>