Major ergonomic changes to control buttons in the MessageView

1) Eliminate the top row Next Message and Previous Message

2) In the bottom buttons for portrait mode, replace Reply and Forward
   with Prev and Next
   Justification: if you're going to be replying or forwarding, you're
   about to embark on a lot of typing, anyway.  Having to hit Menu
   isn't an undue burden.

3) Prev goes to the adjacent *older* message, next goes to the
   adjacent *newer* message

4) In the bottom buttons for landscape mode, added Prev and Next as the
   outer buttons

5) Add Account option for hiding the bottom buttons when the keyboard
   is available.

Also, temporarily (hopefully!) reverted the instant delete from the
MessageView because it isn't working right.  The method here is slower
but safer.