Big update to remove deprecation warning from Qt 5.15.0

That is need on MacOS build as travis log is bigger than 50k lines
which breaks travis rules
And by the way deprecations are real
All file contains the same modification replace 0 to Qt::WindowFlags() when needed
as the class needs to be instantiated

Signed-off-by: vejmarie <jmverdun3@gmail.com>

src/Gui/DocumentModel.cpp

@@ -628,7 +628,7 @@ Qt::ItemFlags DocumentModel::flags(const QModelIndex &index) const
     //    return Qt::ItemIsEnabled;
     //return QAbstractItemModel::flags(index);
     if (!index.isValid())
-        return 0;
+        return Qt::ItemFlags();
     return static_cast<DocumentModelIndex*>(index.internalPointer())->flags();
 }