Fix a .qetz that cannot be opened after only a folio's title changed

With a .qetz and lazy folios (both the default), changing only the
title, author, date or revision of a folio holding a picture, while that
folio was not displayed, then saving, gave a .qetz that refused to open:
'picture "images/<sha>.png" is missing'.

Such a folio is saved from its items as read, with its new header
(Diagram::deferredXmlWithHeader()). Read from a .qetz, its <image> names
its picture's file (qetz-file) instead of holding the picture, and
QetContainer::split() takes out only pictures held inline: the file was
never written, while the folio still named it. The folio now gets its
pictures written in, from the bytes the project keeps
(QETProject::containerPicture()), as a built folio writes them.

On master, a picture on folio 2 of grafcet, iso_sfc_example, 741,
tremie_vibrante or Projet_vierge is lost this way; found on three forum
projects by comparing lazy saves with saves of every folio built. Saving
as .qet was not affected.

New tst_qetzheaderpictures (grafcet.qet: picture on folio 2, saved as
.qetz; its title changed with folios built when needed; reopened with
its picture and title): fails without this change ("picture ... is
missing"). ctest 107/107.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/container/qetcontainer.cpp

@@ -208,6 +208,13 @@ QString QetContainer::pictureFile(const QDomElement &image)
 	return image.attribute(ImageFileAttribute);
 }
 
+void QetContainer::inlinePicture(QDomElement image, const QByteArray &bytes)
+{
+	image.removeAttribute(ImageFileAttribute);
+	image.insertBefore(image.ownerDocument().createTextNode(QString::fromLatin1(bytes.toBase64())),
+					   image.firstChild());
+}
+
 /**
 	@brief QetContainer::ownUuids
 	@return true if every folio, symbol and wire of @p project has a uuid