Add two new properties to SheetCell.idl "FormulaResultType2" and
"CellContentType", because the "FormulaResultType" is returning the
wrong value (com::sun::star::table::CellContentType instead of
com::sun::star::sheet::FormulaResult). Also documeted the curiosity.

Change-Id: Icc6538e155ba27fb9d097d8790ac9b4b230c1446
Reviewed-on: https://gerrit.libreoffice.org/48367Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jens Carl <j.carl43@gmx.de>