SfxHint: convert home-grown RTTI to normal C++ RTTI

Also note that I fixed a bug in SvxFontMenuControl::Notify
where the if statement had the check the wrong way around.

Change-Id: I611e8929c65818191e36bd80f2b985820ada4411
Reviewed-on: https://gerrit.libreoffice.org/11147
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>