cmake compiler flag cleanup, enable many clang warnings and fix them

See merge request !63

Fix danging reference bug caused by halfedge().from_vertex().

See merge request !62

NormalAttrib: default props to 0 instead of undefined

See merge request !61

Implement type check in assignProperties()

See merge request !60

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

cppcheck fixes

See merge request !59

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.