[imp] consistent terminology between IMP and DIMP / MIMP

[Liam Hoekenga]

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.

Liam