Adds reserved keywords virtual.

[christianparpart]

Closes #6040

Simply adds those keywords to the reserved-keywords list, adjusting documentation and ChangeLog to it.

There is a /docs/050-breaking-changes.rst but none for 0.6.0 yet, at lest I didn't find any changelog in the docs/ folder for 0.6.0 just yet - maybe someone can give me a hint in case I overlooked that, so I can adapt that.

[ekpyron]

This should target the develop_060 branch. Did we actually agree on copy? I think the naming isn't even clear yet between copy, copyof and clone and this is probably postponed until 0.7 anyways, so do we need to reserve it now?

[chriseth]

I don't think we should reserve copy at the current time.

[christianparpart]

Okay, so only virtual then.

[ekpyron]

I'd be fine with merging this - although we will actually implement virtual for 0.6.0 anyways (at least I hope we will :-)), so we might as well skip this and just add it as real keyword once we do that.

[christianparpart]

I'd be fine with merging this - although we will actually implement virtual for 0.6.0 anyways (at least I hope we will :-)), so we might as well skip this and just add it as real keyword once we do that.

I'm just walking through some tickets in the ToDo lane. I know that @Marenz is working on override, and I think that this doesn't interfere. In case we want to implement some virtual-semantics (given enough time before [special-date-here] 0.6.0 release, then we can still do so :-)