tests: add a helper for fully syncing empty mailbox A
LibMailboxSync::helperSyncANoMessagesCompleteState(): let the caller provide UIDNEXT/UIDVALIDITY
Add a (failing) test for live updates to the mailbox
Prevent UID FETCH requests for zero UIDs

The issue with preload shall be sorted out later on.
Add a FIXME item for improving message preload

Thanks to Timo for brainstorming.
Enable password authentication also for connections via local process
tests for envelope preload
Gui: allow access to damaged multipart/signed messages

Looks like certain MLs could be configured in a brain-damaged way to strip the
actual cryptographic signature from any message. This results in
multipart/signed messages with just one MIME part. This commit enables the
PartWidget family to render these messages, picking up the first part of such
messages. Better than nothing.
Simplify the Model::Task into a simple enum
Completely remove the ParserState::commandMap
quick-n-dirty fix for the busy cursor
Add a visual clue to mailboxes which contain recent messages
Consult the Trojita's homepage for updated versions

This check for updates can be disabled by setting the app.updates.checkEnabled
config key to false. The webserver is set to log the remote IP address, current
version of Qt and the application version. No private information (like account
names or kind of the IMAP server) are collected.
Add GUI settings for configuring the check for updates
Test that the envelopes are correct
Make sure there are no more message in the mailbox
Verify various message counts
Test a new message arrival
Doxygen++
Fix warning