readonly make cells not selectable

[gdementen]

(discovered in promes_output)

Our DataArrayModel.flags method is slightly buggy:

• when readonly, we need the flag ItemIsSelectable. When the cell is editable, this is (apparently) implied, but specifying it explicitly wouldn't hurt
• It would be more correct to return NoItemFlags for invalid indices

References:

• https://doc.qt.io/qt-5/modelview.html#2-5-the-minimal-editing-example
• https://doc.qt.io/qt-5/qabstractitemmodel.html#flags
• https://doc.qt.io/qt-5/qt.html#ItemFlag-enum