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/qgimanager.cpp

@@ -74,9 +74,11 @@ void QGIManager::release(QGraphicsItem *qgi) {
 	Demande au QGIManager de gerer plusieurs QGI
 	@param qgis QGraphicsItems a gerer
 */
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 void QGIManager::manage(const QList<QGraphicsItem *> &qgis) {
 	foreach(QGraphicsItem *qgi, qgis) manage(qgi);
 }
+#endif
 
 /**
 	Indique au QGIManager que pour chaque QGI fourni, une reference vers celui-ci
@@ -85,9 +87,11 @@ void QGIManager::manage(const QList<QGraphicsItem *> &qgis) {
 	sur la scene de ce QGIManager, alors il sera detruit.
 	@param qgis QGraphicsItems a ne plus gerer
 */
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 void QGIManager::release(const QList<QGraphicsItem *> &qgis) {
 	foreach(QGraphicsItem *qgi, qgis) release(qgi);
 }
+#endif
 
 void QGIManager::manage(const QVector<QGraphicsItem *> &items) {
 	for (const auto &qgi : items) {