Cleanup

See merge request !58

This reverts commit 6ff4e080.

Properties: Store and compare type names instead of relying on dynamic_cast

See merge request !57

Implement/improve mesh assignment/copying

See merge request !56

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

Implement generic mesh::request_property templated on entity type.

See merge request !55

Cleanup and small features

See merge request !54

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.

Properties: use partial template specialization.

See merge request !53

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.

Refactor using Entity tags

See merge request !52

Renamed T.cc files to T_impl.hh to get rid of IDE problems not showing them.

See merge request !51

PropertyPtr: clarify public interface by only exposing the necessary shared_ptr methods (cf #10)

See merge request !50

Delete broken operators

See merge request !49

Fix warnings

See merge request !48