Recognise seqhf_1 when loading sequential numbers saved the old way

Before the <sequentialNumbers> element, an element's or conductor's
sequential numbers were saved as the attributes sequ_1, sequf_1,
seqt_1, seqtf_1, seqh_1 and seqhf_1. The readers check for one of them
to take the old route, and all three lists had the same slip: sequf_1
twice, seqhf_1 never (Element::fromXml, Conductor::fromXml, and
readSequence() in the project database). A file whose only old
sequence was the hundred-folio one took the new route, found no
<sequentialNumbers>, and lost it; the database built its labels from
an empty sequence instead of refusing the fast path as it does for the
other five attributes.

Name seqhf_1 in the three lists.

No file-format change: nothing is written differently, and a file with
any of the other five attributes loads exactly as before.

Tests: tst_legacysequentialattributes runs the binary's --resave on a
fixture whose element and conductor carry only seqhf_1 (and a second
pair carrying sequ_1 as a control) and reads the <sequentialNumbers>
written back. The two seqhf_1 cases fail on master.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: Beat Hangartner <beat@hangartners.ch>

sources/dataBase/projectdatabase.cpp

@@ -236,7 +236,7 @@ struct DocumentElement
 	//the attributes files written before <sequentialNumbers> carry.
 bool readSequence(const QDomElement &item, autonum::sequentialNumbers *sequence)
 {
-	for (const char *name : {"sequ_1", "sequf_1", "seqt_1", "seqtf_1", "seqh_1"}) {
+	for (const char *name : {"sequ_1", "sequf_1", "seqt_1", "seqtf_1", "seqh_1", "seqhf_1"}) {
 		if (item.hasAttribute(QLatin1String(name))) {
 			return false;
 		}