Use universal references for passing callables in QtConcurrent

class MyFilter {
  bool operator(...) &&  // rvalue-ref overloaded
  {
    // when I am a temporary object, I can do something differently
  }
  bool operator(...) &  // lvalue-ref overloaded
  {
    // here, I am an lvalue
  }
  // ...
};
auto future1 = filter(sequence, MyFilter());  // [0]
MyFilter lvalueFilter();
auto future2 = filter(sequence, lvalueFilter);  // [1]

// [0] might differ from [1] in the filter logic.