* notmuch: add placeholder since it has native thread support
* maildir: Return false since it does not have native thread support

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

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

* Implement a simplified version of the REFERENCES algorithm
* Remove FormatThreads function
* Instead of acting on all threads, handle each thread independently

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>

Configure an oauthbearer source without a token_endpoint
parameter would panic due to nil pointer dereference

  Example
    source=imaps+oauthbearer://frode.aa%40gmail.com@imap.gmail.com:993
    source-cred-cmd=pass oatuh2 frode.aa@gmail.com

token_endpoint is not required as it will use the provided
password as access_token when it is not set

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.