types: Add STL-compliant Hash functor for Qt types

types: Add STL-compliant Hash functor for Qt types

In order to use Qt types with std::unordered_set (and map), a
compatible hash function needs to be provided. Provide a generic
Hash<T> functor that just uses the qHash() function for the given
type.

This can be used as follows:
    std::unordered_set<QString, Hash<QString>> set;