fdo#76260 - don't allocate and free std::strings on each element.

This debug code is quite expensive; 2.5% of document load time;
Avoid the pointless heap allocation, and punt the rest to fdo#80798.

Change-Id: I499b3cf48867d5ed0f2b33b9d12f1c128c993aae
DLG_MM_DBTABLEPREVIEWDIALOG and DLG_MM_MAILBODY conversion to .ui

Change-Id: Ie00c75e54918e51068ab69c5ea9047d0503b5b87
Reviewed-on: https://gerrit.libreoffice.org/10004
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>