Make wl-highlight-message use the citation depth for choosing the face.

Rather than using a stateful algorithm for choosing arbitrary but
distinct faces for the various citation prefixes in the message,
this uses the citation depth for plain citations and an arbitrary
hash of the attribution name for SC-style attributions.
Implement draft highlighting using jit-lock-mode.

This uses Emacs' 21 native highlighting mechanism, and is enabled
by default if available.
Don't recompute in-header in wl-draft-default-jit-highlight.

This was wrong, since point may have changed by then, and caused
mis-highlighting of the first line if it was quoted.
Merge branch 'jit-lock' of https://github.com/jech/wanderlust into t/jit-lock
Merge branch 'master' into t/jit-lock
Merge branch 'master' into t/jit-lock
Merge branch 't/jit-lock'