hide / re-locate legacy export formats

Background: When a user uses save-as, we present a list of potential export filters in the drop-down, many of which are obsolete and unhelpful. We should add a check-box or other method to the file-picker (defaulting to on) to hide these. We should do this for the OpenOffice 1.0 pre-ODF formats, and legacy MS Office formats (e.g. Word 95 and 6.0 formats). The file pickers are implemented per platform, and live in: fpicker/source/ - for gtk+ perhaps we could use the file type expander for the legacy formats. The UNO interface to export filters is built around UnoFilterEntry records which is just a string pair; misc code-pointers:

    sfx2/source/dialog/filedlghelper.cxx (addFilters) - gets data from
   + filters/source/config/cache/filterfactory.cxx (createSubSetEnumerationByQuery)
      + which gets its data ultimately from filter fragments
        eg. filter/source/config/fragments/filters/StarWriter_5_0.xcu
      + which will need a new Flag annotation I suspect.
    sfx2/source/dialog/filtergrouping.cxx (appendFiltersForSave)

Possibly we can mangle some annotation for the filter into the filter string - this would be the lowest touch change.

Skills: C++, basic UNO, grokking several files at once.