make attribute table a shared_ptr and get HTMLAttrs that belong
to it to add a reference to it while they exist

Change-Id: I2d3ba433700d4a98fd466b38a495ed5a7adae020
Reviewed-on: https://gerrit.libreoffice.org/46449Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>