Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

FormatThread performs dfs on a thread tree. For every node in the tree,
a callback function is called with a thread and a format string for
that thread. The format string is to be used when displaying the full
thread tree.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

* Import the go-imap-sortthread library
* Add sortthread client to imapClient in worker
* Add handleDirectoryThreaded, which uses the go-imap-sortthread, and
  converts the results to the aerc thread type

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

* Add threading-enabled config option
* Add DirectoryThreaded and FetchDirectoryThreaded types to control path
* Add generic thread type for all backends to use

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

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.