mu4e-compose: overhaul message composer

This is a new composer sub-system, which more closely follows
Gnus/message; in particular, we let gnus take care of creating draft
messages.

This reduces code size and allows us to re-use some of the more advanced
features.

There are a number of user-visible changes:

- `mu4e-compose-resend' directly uses `message-resend', so there is no
  more editing phase (which used to be a bit broken with re-encoding
  MIME messages.

mu4e/meson.build

@@ -43,7 +43,6 @@ mu4e_srcs=[
   'mu4e-contacts.el',
   'mu4e-context.el',
   'mu4e-contrib.el',
-  'mu4e-draft.el',
   'mu4e-folders.el',
   'mu4e.el',
   'mu4e-headers.el',