The comment message widget is supposed to be visible after the user
edits an existing comment. However the current logic makes it visible
even after opening two archives with a comment in a row:

1. The first time the message widget is hidden as expected, because it
   was explicitly hidden in the Part ctor and `m_commentBox->show()` in
   `slotShowComment()` still leaves it hidden.

2. The second time, the message widget is breafly shown is
   `slotCommentChanged()` because `m_commentView->toPlainText()` is empty
   (it gets cleared before opening the second archive). The message
   widgets stays visibile because it's not hidden anywhere else.

This patch fixes the issue by preventing that the message widget gets shown
*without reason* in the first place.