This allows backends which can't always be compiled due to missing
dependencies (say libnotmuch) to be compiled conditionally with buildflags.

When a directory is opened, start watching its "new" subdirectory for
incoming messages using the fsnotify library. When creation events are
detected, run the Unseen routine to move the message from new to cur and
add new UIDs to the store, updating the UI's list of directory contents
as we go.

Email headers can be encoded with different charsets, which is signalled
using a special character sequence. The go-message package provides two
different methods for accessing header values, Get(key) (actually
inherited from the embedded textproto.Header) which returns the raw
header value and Text(key), which returns the header's value decoded as
UTF-8.

Before, in the maildir backend, we were using the Get method which
sometimes resulted in encoded headers being displayed in the UI. This
patch replaces the incorrect usage of Get() with Text().

Create a delivery in the destination directory with the content of the
source message.

Add the initial implementation of a backend for Maildir accounts. Much
of the functionality required is implemented in the go-message and
go-maildir libraries, so we use them as much as possible.

The maildir worker hooks into a new maildir:// URL scheme in the
accounts.conf file which points to a container of several maildir
directories. From there, the OpenDirectory, FetchDirectoryContents, etc
messages work on subdirectories. This is implemented as a Container
struct which handles mapping between the symbolic email folder names and
UIDs to the concrete directories and file names.

    imaps+oauthbearer://user:token@host?token_endpoint=...

 - the config Source password is used as access token if
   no token_endpoint parameter is set
 - the config Source password is used as refresh token if
   token_endpoint parameter is set, and used to exchange
   with an access token

The implementation has only been tested with Gmail.

    source = imaps+oauthbearer://{username}:{refersh_token}@imap.gmail.com:993? \
    client_id=XX&\
    client_secret=XX&\
    token_endpoint=https%3A%2F%2Faccounts.google.com%2Fo%2Foauth2%2Ftoken

client credentials created with

    https://console.developers.google.com/apis/credentials

refresh token created with

    https://github.com/google/gmail-oauth2-tools/blob/master/python/oauth2.py

rel: https://todo.sr.ht/~sircmpwn/aerc2/42

Before, we were using several IMAP-specific concepts to represent
information being displayed in the UI. Factor these structures out of
the IMAP package to make it easier for other backends to provide the
required information.