fdo#75757: remove inheritance to std::vector

from SwLabRecs, by typedef'ing it as boost::ptr_vector<SwLabRec>.

This is also a supposed fix for possible memory leaks in
SwLabDlg::_ReplaceGroup(), since erasing elements did not
delete pointers in the original code.

Change-Id: I1c8a0393cb5bdcfca3408e9c8dc770d306f6d334
Reviewed-on: https://gerrit.libreoffice.org/11597
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>