QScopeGuard: Make it a non-template class (7.0)

1. Make QScopeGuard a non-template class.
2. Make private T m_func field of a std::function<void()> type, initialized to {} by default.
3. Get rid of bool m_invoke field, it's enough to check whether m_func is set or not.
4. Provide a c'tor taking std::function<void()> type.
5. Optionally provide a template c'tor taking T and assigning it to m_func.
6. Provide a default c'tor.

Rationale:

Currently it's not possible to do:

bool condition = ...;
const auto cleanup = condition ? qScopeGuard([] { qDebug() << "Here"; }) : QScopeGuard();

since there is no default c'tor. Even when doing artificial:

const auto cleanup = condition ? qScopeGuard([] { qDebug() << "Here"; ) : QScopeGuard([] {});

the compiler complains about the not matching types for the left and right operands of "? :" operator.

Besides, there is really no reason for this class to be a class template.