Special case Lua copy-construction of trackable instances

Special case Lua copy-construction of trackable instances

This fixes an crashing issue with ArdourUI.SelectionList a bug
introduced in 6dc3bdf252c and 35dcd46d7d.

Since removal of the special cases in 35dcd46d7d, when using
a C-pointer in a std::list<>,
  std::list<class*>::push_back(TypeListValue)
TypeListValues<>'s Head was expanded to  "class*& const"
implied by void ::push_back(const T& value);

This resulted in lifetime issues with a classes that derive
from sigc::trackable (e.g. Ardour's Selection).
The reference leaves scope and isn't duplicated when it is pushed back
to the std::list<>.

The script scripts/select_every_2nd_region.lua crashed because entries
in the SelectionList were no longer valid.

Previously (before 6dc3bdf252c) TypeListValues explicitly
copy-constructed the value to work around the lifetime issue.
This new solution bypasses the issue by directly using the c-pointer
without dereferencing it.