Fix the remaining Qt6 compile blockers

Clears the last five spots that stop master from compiling against Qt6
(all mirrored from the proven qt6-build line):

- qgimanager.h/.cpp: in Qt6 QVector is an alias for QList, so the
  deprecated QList overloads of manage()/release() collide with the
  QVector ones (same signature). Keep them on Qt5 only.
- diagramview.cpp: one unguarded QTextStream::setCodec() call (removed
  in Qt6, which defaults to UTF-8).
- print/projectprintwindow.cpp: QApplication::desktop() was removed in
  Qt6; use QWidget::screen() there, keep the old path on Qt5.
- titleblocktemplate.cpp: QDomDocument::setContent() returns a
  ParseResult in Qt6 whose operator bool is explicit; static_cast keeps
  the bool initialization working on both.

With these, master configures and builds to a running binary with
Qt 6.11 (mingw, BUILD_WITH_KF5=OFF); every change is guarded or
dual-safe, the Qt5 build is unaffected.

sources/diagramview.cpp

@@ -1355,7 +1355,9 @@ void DiagramView::createTemplateFromSelection()
 	QFile file(full_path);
 	if (file.open(QIODevice::WriteOnly | QIODevice::Text)) {
 		QTextStream out(&file);
-		out.setCodec("UTF-8");
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)	// ### Qt 6: remove
+		out.setCodec("UTF-8");	// Qt6 QTextStream defaults to UTF-8
+#endif
 		out << macro_doc.toString(4);
 		file.close();
 		qDebug() << "Template successfully saved to:" << full_path;