Make Coverity ignore warnings about QDialog::exec

Coverity complains about the following code sequence:

QDialog dlg(this); // also without parent
if (dlg.exec()) { // warning: Free of address of expression (BAD_FREE)
   ...
}

because QDialog::exec() might call

    bool deleteOnClose = testAttribute(Qt::WA_DeleteOnClose);
    // ...
    if (deleteOnClose)
        delete this;

WA_DeleteOnClose is never set in the cases that Coverity warns about. It can be, in which case it would be a programming error that Coverity should warn about.

It might be possible to provide a modeling file to train Coverity that this code patternis not problematic. Failing that, we might need to find another way to dismiss those warnings.

Coverity ID 265281 and more.