Fix miscellaneous memory leaks

thumbnailview.cxx: ::ImplInitSettings is called from various
places other than the constructor, so mpItemAttrs must be
freed each time
gloactiongroup.cxx: Must call g_list_free on list returned by
g_hash_table_get_keys

Change-Id: Id5a592520987a1596debdbc48bfa171a537e400b
Reviewed-on: https://gerrit.libreoffice.org/11638
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>