If the document changes the comment state from the old document should
be replaced with that of the new document. To do this the comment
manager is destroyed, but the corresponding comment sidebar wasn't
destroyed yet, which resulted in the comment state from the old document
still being visible for the new document.
This commit fixes the issue by hiding the comment sidebar if the comment
manager is destroyed. Note that hiding the comment sidebar effectively
destroys all its state, and we already set the annotation mode to "none"
on document change so we don't want to keep showing the comment sidebar
anyway.
