Add overload for QFileDialog::open() that accepts function pointers

Many callback functions that used to accept names of slots or Q_INVOKABLE methods now accept function pointers too:

• QHostInfo::lookupHost()
• QTimer::singleShot()
• QMetaObject::invokeMethod()

For completeness, it would be nice if QFileDialog::open() followed suit.

P.S. The 4 functions listed above are all that I'm aware of. Do any other such functions exist?