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.

This removes old aborted deliveries from the tmp directory.

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.