In the search of the help system, make the number of topics to be
searched for before the filtering is applied configurable, so that it
can be set in the "plugin_customization.ini" file.

If the number is too small, hits may be missed, e.g. when searching in a
topic and its subtopics. The higher the number, the slightly worse the
performance. The default is 1,000 and the maximum number of hits
displayed to the user is 500, which means that if more than 500 of the
hits before filtering are outside of the search scope, some hits will be
missed.

See also:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=357857