Fix Flag Warnings

[RobertBColton]

First, the Qt::ItemFlags constructor for int was deprecated, so neither 0 nor nullptr were correct for invalid model indexes. The correct replacement is Qt::NoItemFlags that is assigned 0 in the enum. Second, apparently I made a mistake when I added the simple about dialog using QMessageBox in that I was passing 0 for the window flags parameter, then later changed it to nullptr because I confused it with the parent parameter. That's not good because it needed the Qt::Dialog flag it gets by default, so I just removed that argument from the call.
https://doc.qt.io/qt-5/qmessagebox.html#QMessageBox-1