Add support for isGroupRow

[brendand]

NSTableView has an isGroupRow delegate which returns a BOOL if the current row should span across all columns and be drawn using a different style than the rest of the rows.

For example:

- (BOOL)tableView:(NSTableView *)tableView isGroupRow:(NSInteger)row {
    BOOL isGroupRow = NO;
    // do something to determine from your model if the current row
    // is a section header (or group row)
    return isGroupRow;
}

It should also prevent selecting of cells in isGroupRow rows.

[brendand]

Updated the sample app so row 0 and row 5 are designated as group rows. Just randomly picked those values.

[Dejal]

Might want to consider supporting floating group rows. (See -addFloatingSubview:forAxis:.) Probably not trivial, though.