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We should not return references to members of possible xvalues.
Handle is lightweight enough that copying is not a problem.

BaseProperty cleanup: explicitly define destructor in its own translation unit, avoid duplicated vtables

implement assigning and moving properties; make ResourceManager move assignment consistent with copy assignment

Simplify property destruction logic and do not keep non-persistent props
around in clearVec(). If a prop is alive but non-persistent, someone
else has a shared_ptr to it, so we will not pull out the rug from under
their feet.

Avoid a lot of code duplication.

Also change constructor _def param to a const&, which was only in
the std::string specialization before, but makes sense everywhere.

Currently, the following API-usage is straight-forward and clean but
unfortunately wrong and dangerous:

`for (const auto heh: mesh.face(fh).halfedges()) { ... }`

This is because the temporary `Face` returned by `face()` reaches the
end of its lifetime, but `halfedges()` returned a reference to a vector that
lives inside that object. Reference lifetime extension does not apply
here.

This change splits behaviour so that in rvalue context (such as
temporaries), we move out the vector instead of returning a reference to
it.