QmlJS: Fix crash when opening JS files.

Now that Contexts are cached, it needs to be copyable. However, the
ScopeChain has a QmlComponentChain member that owns resources and didn't
have a correct copy constructor or copy assignment operator.

I've made QmlComponentChain non-copyable and store a shared pointer to
an instance instead, as it will generally not change for a given
context.

Reviewed-by: Lasse Holmstedt