CXX-Qt-Lib: Add CXX-compatible templates for make_unique, make_shared, etc.

[LeonMatthesKDAB]

To complete our list of templated constructors in cxx-qt-lib/common.h.

We probably want to have at least:

• construct
• make_unique (added in Add QPainter support #779 - to be released with 0.7)
• make_shared
• new/new_ptr (contributed in add new_ptr template #905 , thanks to @knoxfighter)
• upcast / downcast helpers
• Consider moving these into cxx-qt rather than cxx-qt-lib

[ahayzen-kdab]

Note this is also something that if cxx-qt-lib is split out could go into cxx-qt or a -common thing later 🤔

[ahayzen-kdab]

See comments in #840, consider having an up and downcast helper

template<typename Base, typename T>
Base & upcast(T& t)
{
   // Maybe add some static_assert that Base is indeed base class of T
    return static_cast<Base&>(t);
}