cleanup: don't shadow the parent class, remove useless redefines
tests: port to LibMailboxSync's native features
IMAP: do not request STATUS of mailboxes which are going to be replaced

The LIST processing will replace the cached mailboxes with completely new
instances, so it doesn't make sense to issue a STATUS for something which is
going to be replaced anyway.

When LIST-STATUS is present, this makes even more sense -- even if the TreeItem
nodes were not replaced, the STATUS response would still arrive, so there is
little point in requesting it explicitly. But that's just a remark for future;
the TreeItemMailbox nodes are for now being replaced when a LIST response
arrives.
Return previously known values when asked for message counts

When a message count is invalidated for some reason (perhaps because the
periodic timer has run), the GUI would not show the number of message at all.
This is suboptimal; in the name of getting a more accurate result in future,
what we get is that the actual value disappears for a while. This change makes
sure that we never forget the old data, even when asked for the new ones.
Do not claim the numbers are fresh when loading from cache

When a list of child mailboxes is loaded from the cache, it used to be shown as
grayed-out to make it obvious that the value might very well be stale. The
recent changes made them appear normal, which might be confusing.

We're cheating in this commit -- the value is marked as loading, so it will be
again shown as grayed-out, but there is no matching network request. Instead,
the code knows that the list of mailboxes will be replaced when the LIST
response arrives.
Merge branch 'bug-332113'

BUG: 332113
REVIEW: 116794