Improve tag handling; update test

[uuf6429]

This isn't a new feature, but rather a small improvement on tag handling.

Here's what changed:

1. hasTag/hasTags code is always the same, so I moved it into a trait (that depends on abstract method getTags).
2. I removed the somewhat redundant PHPDoc comments, in favour of the one provided by the interface
3. ExampleTableNode now implements TaggedNodeInterface and the new trait - I couldn't see why it wouldn't have that in the first place
4. I've updated a test to cover point 3.

It would have been nice to move getTags to the trait too, but at this PHP level we can't define abstract properties or property requirements, so I think this is the safest and cleanest approach at the moment.