[imp] consistent terminology between IMP and DIMP / MIMP

[Chuck Hagenbuch]

Quoting Liam Hoekenga <liamr at deathstar.org>:

> To create a new message in IMP, one uses "New Message"
> To create a new message in MIMP and DIMP, one uses "Compose"
>
> I think that the three views should use the same terminology for all
> features shared by all views.  The "New Message" / "Compose" variation
> is the only one I've noticed, but they should be checked for consistency.

We changed from Compose to New Message in IMP for consistency with  
other apps (New Task, New Event, etc.). I think we should follow  
through in MIMP and DIMP also.

I'll do so within a day or so unless there are any objections.

-chuck