Virtually all callers of this function (will) pass rvalues, so take
the std::function by value (reaping C++17 guaranteed copy elision) and
std::move() into the member variable ("perfect sink").
Like for many owning types, moves are much cheaper than copies for
std::function, because the external state is merely tranferred between
objects, and not copied.
Amends e560adef21.
Pick-to: 6.7
Change-Id: I269b54e51ba09ac595ac4e4f255209778819adad
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
a564618838