Comment 4 for bug 1416810

> It doesn't appear until an application registers itself.

Judging from simple testing, that is exactly where libmessaging-menu0 is needed for:
- installed thunderbird and indicator-messaging
- started thunderbird -> messaging-indicator does not appear (although the indicator-plugin lists it now as known indicator)
- installed libmessaging-menu0 -> messaging-indicator does appear
- removed libmessaging-menu0 -> messaging-indicator still there