Speed up opening .qetz projects with large pictures

A .qetz keeps each folio picture as a PNG file. On opening, join() put
every picture back into the document as base64 text, and the folio then
decoded that text again, twice. On a project with 20 pictures of 7 MB
(140 MB of PNG, the shape of the project on PR #1462) that cost about
8 of the 12 seconds the open took:

  QDomDocument::createTextNode() on the base64   3.9 s
  DiagramImageItem::fromXml(), base64 decoded    3.9 s (2 x 1.9 s)
  the PNG decoding itself                        0.3 s

join() now takes an optional map. Given one, each picture stays a file:
the <image> keeps its qetz-file attribute and the bytes go in the map.
QETProject keeps the map for as long as the project is open, and
Diagram::fromXml() hands a picture's bytes to the new
DiagramImageItem::fromXml(element, picture). Kept, not cleared after
the load, so a folio built later finds its pictures too; the bytes are
shared with the PNG cache of the pictures built from them, so this costs
almost no memory. readDocument() without a map, as the automatic
numbering import uses it, still gives the whole document.

fromXml() of a .qet picture also decodes its base64 once instead of
twice: the first text node, which the PNG cache already used. e.text()
also collects <image_base>'s text, but loadFromData() stops at the end
of the first picture, so the picture shown is the same.

Measured, same file, 3 runs each, offscreen:
  .qetz  11.8 s -> 3.8 s, peak memory 1034 MB -> 655 MB
  .qet   15.7 s -> 13.6 s
Resaving either file with either binary gives the same bytes, a cropped
and colour-keyed picture included, and every folio exports to the same
PNG pixels.

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

sources/qetproject.cpp

@@ -276,6 +276,20 @@ QUuid QETProject::uuid() const
 	return m_uuid;
 }
 
+/**
+	@brief QETProject::containerPicture
+	The folio pictures of a .qetz are kept as bytes beside the document,
+	not as base64 text in it (QetContainer::join()), for as long as the
+	project is open: a folio built after opening still finds its pictures.
+	They cost almost no memory, sharing their bytes with the pictures
+	built from them.
+	@return the picture in @p file, or an empty array when there is none
+*/
+QByteArray QETProject::containerPicture(const QString &file) const
+{
+	return m_container_pictures.value(file);
+}
+
 /**
 	@brief QETProject::derivedItemUuid
 	A uuid for an item of this project that was saved without one, the same
@@ -438,9 +452,10 @@ QETProject::ProjectState QETProject::openFile(QFile *file)
 	// QetContainer::parse() does exactly that, for a .qet and for each part
 	// of a .qetz alike.
 	QString read_error;
-	if (!QetContainer::readDocument(content, &xml_project, &read_error))
+	if (!QetContainer::readDocument(content, &xml_project, &read_error, &m_container_pictures))
 	{
 		qWarning().noquote() << "Unable to read" << fi.fileName() << ":" << read_error;
+		m_container_pictures.clear();
 		if(opened_here) {
 			file->close();
 		}