Add some help content to test-passive.oxt

...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 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`.