Fix hiding a message while some citations are shown in notmuch-show view.

Fix hiding a message while some citations are shown in notmuch-show view.

Before the change, message and citation invisibility overlays
conflicted: if some citation is made visible and then the whole
message is hidden, that citation remained visible.  This happened
because the citation's overlay has an invisible property which
takes priority over the message overlay.  The message
invisibility spec does not affect citation visibility, it is
determined solely by the citation overlay invisibility spec.
Hence, if citation is made visible, it is not hidden by message
invisibility spec.

The patch changes citation overlay invisibility property to be a
list which contains both the citation and the message
invisibility specs.  This makes the citation invisible if either
of them is added to the `buffer-invisibility-spec'.  Note that
all citation visibility states are "restored" when the message
hidden and shown again.