https://ericniebler.com/2013/08/07/universal-references-and-the-copy-constructo
describes the issue solved by this commit. The blog post recommends
constraining the forwarding-reference constructor. But that solution
would require much more code than added in this commit. Especially
because the constraint would have to be expanded to support template
parameter pack.

The deleted constructor overloads prevent surprising and undesirable
compilation of the following two code snippets:
    ScopedDialog<QDialog> s;
    ScopedDialog<QDialog> s2(s);
and
    const ScopedDialog<QDialog> s;
    ScopedDialog<QDialog> s2(std::move(s));
They were compiled successfully because the forwarding-reference
constructor was a better match than the deleted copy and move
constructor respectively (due to const-qualifier differences). In the
forwarding-reference constructor invoked in these snippets, a reference
to ScopedDialog `s` was implicitly converted to QDialog* and passed as a
parent to `s2`'s new QDialog.

The final specifier prevents surprising and undesirable compilation of
the following code snippet (as well as the 3 other const and
rvalue-reference variations of it):
```
template <typename T> class ScopedDerived : public ScopedDialog<T> {};
void f() {
    ScopedDerived<QDialog> s;
    ScopedDialog<QDialog> s2(s);
}
```

A volatile ScopedDialog (which does not make sense, by the way) cannot
be copied or moved for another reason: passing volatile ScopedDialog as
`this` argument to `operator DialogType*() const` discards qualifiers.