Add STL-style assign() overload sets to Qt containers.

Qt container are lacking the assign() overloads of STL containers:

• https://en.cppreference.com/w/cpp/string/basic_string/assign
• https://en.cppreference.com/w/cpp/container/vector/assign

These functions are a convenient way to assign new data while reusing the allocated memory:

std::string s;
s.assign(p1, n1); // probably allocates
s.assign(p2, n2); // may not allocate, if n1 >= n2

QByteArray s;
s = QByteArray(p1, n1); // allocates
s = QByteArray(p2, n2); // allocates, too, even if n2 <= n1

The STL views (string_view) don't have the assign() function, so it's basically the owning Qt containers:

• QByteArray
• QString
• QList
• QVLA

We should also look around the Qt code to find potential users of the new functions.