This ensures that the directory info is up to date on events in the
maildir worker. This also sets up the initial dirinfo for other
directories and updates them when using built-in commands.

FS events are still only watched for the selected directory. This should
be changed in a future patch to watch other directories too in order to
cover UI updates for folders when an event occurs in a non-selected
folder.

When we fail to read a single message we don't need to fail the whole
search, just log the error and we can still get results back.

Exposes the notmuch tags accordingly, stubs it for the maildir worker.

This changes the search flags for maildir and imap backends.

They now no longer use -t for searching all text. This seems to make
more sense as being the targeted recipient. I have similarly added Cc
for -c. The text search now resides under -a for all text.

There is a command and config option. The criteria are a list of the
sort criterion and each can be individually reversed.

This only includes support for sorting in the maildir backend currently.
The other backends are not supported in this patch.

This populates the directory info model properly when requested,
allowing the fields to be relied upon elsewhere.

This also sends the dirinfo when new messages come in.

Basic searching is supported with the following:
- read messages
- unread messages
- from addresses
- text in body
- text in subject
- text in all

The implementation loops through all messages in the selected directory.
It tries to be smart by detecting which parts of each message the search
query needs to use and only loads these from the filesystem.

Things like FetchEntityPartReader etc can be reused by most workers
working with raw email files from disk (or any reader for that matter).

This patch extract that common functionality in a separate package.