Get the attributes of the value stored at (row, col) in this, which is an ORed value of ValueAttribute flags.
As a special case, if row is -1, then the attributes returned correspond to a "would be" value if a row was added to
this.
| this |
a DataModel object |
| col |
a valid column number |
| row |
a valid row number, or -1 |
|
the attributes as an ORed value of ValueAttribute |