Let the list of bookmarks in the 'Edit bookmarks' dialog be resizable

Change-Id: I09279f4d35238e6ab2d7b3e27e711dfe7fd74211
Reviewed-on: https://gerrit.libreoffice.org/7399
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>