Problem reports are too noisy

This re-works problem reporting to treat being offline, or encountering connectivity, login or TLS cert errors as a persistent status rather than as isolated problem reports, so that if/when service is restored any info bars indicating this can be automatically revoked. All service problems for all accounts are now squashed into a single info bar, retrying will retry all affected accounts. Same for auth problems, except that auth problems (and cert problems, when the fix for #20 (closed) lands) will only be retried one at a time. Geary will now also display an info bar when offline, which will also be automatically revoked when connectivity returns and handle account opening problems using problem reports, rather than causing the app to exit.

Also tidies up some engine internals so handling auth errors is handled in the same way (and eventually cert errors), fixes some issues handling IMAP commands that fail (e.g. login).

Fixes #17 (closed)