Add QHash::try_emplace

Our best functionality currently to insert-if-not-already-exists and then know about it is something along the lines of

oldsize = hash.size();
auto &obj = hash[key];
if (oldsize != hash.size())
  obj = ...;

This is because none of our current functionality returns any indication whether there was a new insertion or not. With tryEmplace/try_emplace we can get this.