This patch fixes one bug in the previous version, cleans up the code, adds
separate wrap-lengths for sending vs. browsing, and adds control over
browse wrap to the prefs page, synchronized with the Edit -> Wrap body
option on the New Message window.

The patch replaces all previous versions; it requires recent cvs, including
Albrecht's `forward' patch; it will conflict with earlier source trees,
such as 1.2.0.

Affected files:

* libbalsa/message.h
- add `flow' argument to various libbalsa_message_*

* libbalsa/mime.[ch]
- in process_mime_part, use `flow' to decide how to wrap lines 
- pass around the `flow' argument

* libbalsa/misc.[ch]
- add 2 public functions and some private support

* libbalsa/send.c
- set the `format=flowed' attribute in add_mutt_body_plain
- pass around the `flow' argument

* src/balsa-app.[ch]
- add some fields to balsa_app

* src/balsa-message.c
- choose how to wrap lines for display

* src/main-window.[ch]
- pass the `flow' option to libbalsa_process_queue
- defines update_view_menu, used to synchronize the view menu to the prefs
window

* src/pref-manager.c
- user control over whether to reflow paragraphs in incoming
`format=flowed' messages, and whether to generate outgoing `format=flowed'
messages
- separate wrap lengths for incoming vs. outgoing
- synchronize the view menu to the prefs window with update_view_menu

* src/save-restore.c
- ditto

* src/sendmsg-window.[ch]
- pass the `flow' option to libbalsa code
- choose how to wrap lines for sending and for display
- catch `Enter' keystrokes to trim trailing spaces, when generating a
`format=flowed' message