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>
(cherry picked from commit 2f2d9e3799d041381565ac1ef6e7a8c8a065871b)