There was a warning:
```
[100%] Building CXX object demo/Polyhedron/CMakeFiles/polyhedron_demo.dir/MainWindow.cpp.o
/home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.cpp: In constructor ‘MainWindow::MainWindow(bool, QWidget*)’:
/home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.cpp:312:11: error: request for member ‘addDockWidget’ is ambiguous
this->addDockWidget(Qt::BottomDockWidgetArea, dock);
^~~~~~~~~~~~~
In file included from /home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.h:8:0,
from /home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.cpp:4:
/home/lrineau/Git/cgal/Three/include/CGAL/Three/Three.h:54:8: note: candidates are: void CGAL::Three::Three::addDockWidget(QDockWidget*)
void addDockWidget(QDockWidget* dock_widget);
^~~~~~~~~~~~~
In file included from /usr/include/qt5/QtWidgets/QMainWindow:1:0,
from /home/lrineau/Git/cgal/GraphicsView/include/CGAL/Qt/DemosMainWindow.h:31,
from /home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.h:7,
from /home/lrineau/Git/cgal/Polyhedron/demo/Polyhedron/MainWindow.cpp:4:
/usr/include/qt5/QtWidgets/qmainwindow.h:166:10: note: void QMainWindow::addDockWidget(Qt::DockWidgetArea, QDockWidget*, Qt::Orientation)
void addDockWidget(Qt::DockWidgetArea area, QDockWidget *dockwidget,
^~~~~~~~~~~~~
/usr/include/qt5/QtWidgets/qmainwindow.h:165:10: note: void QMainWindow::addDockWidget(Qt::DockWidgetArea, QDockWidget*)
void addDockWidget(Qt::DockWidgetArea area, QDockWidget *dockwidget);
^~~~~~~~~~~~~
```
That is strange, because the call is not ambiguous at all: all three
candidates have different numbers of parameters!
Anyway, the workaround is an explicit qualified call to
`QMainWindow::addDockWidget`.
This allows the c3t3_item to override it and return an empty list, so it ignores the selection behavior of the group and keeps it possible to apply an operation only to the c3t3 when it has children like visible tets.
Now that we require CMake>=3.1, we can forget a lot of old CMake policies.
`CMP0053` `OLD` behavior is still needed, because of a bug in Qt5 CMake files, but it will
not be set with CMake>=3.10. Let's assume that a recent CMake version means
a recent Qt5 version.