Add dereference operator to ComponentHandle

[lethal-guitar]

This makes it possible to write code like this:

auto& componentRef = *someEntity.component<Position>();
componentRef.x += 2;

This was already possible previously, but required calling .get() on the component handle in addition to dereferencing. Having this operator also makes the component handle more consistent with common smart pointer types, since it already has the -> operator.

I wasn't sure whether it makes sense to add a test for something so trivial, but I did for now.