[imp] consistent terminology between IMP and DIMP / MIMP

[Chuck Hagenbuch]

Quoting Liam Hoekenga <liamr at deathstar.org>:

> Another piece of text to change..  after you send a message in IMP, if
> you've got the confirmation screen turned on, you see two options -
> "Compose another message" and "close window".  I'd argue that  the "New
> Message" term should be used here too.

Makes sense to me. Changed, thank you.

-chuck