mu4e-compose: split off parts in mu4e-draft; rework

Move draft-creation part of message-composition to a new file
mu4e-drafts

However, this is more than just some refactoring: rework the
message (draft) setup as such to ensure we always get a (non-active,
live) buffer back from the composition process.

This also fixes

(progn
  (compose-mail "test@example.org" "Test")
  (insert "TEST"))

as mentioned on the mailing list.

Fixes #2699

mu4e/meson.build

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