XD-767 & XD-768 mail source & sink

Adds "mail" and "imap" sources (pending a more elegant solution with XD-785)
Adds "mail" sink

Adds support for shell integration tests.
For some reason, the imap idle test succeeds even though there are stack traces about IDLE being not supported. Don't know if this is because the server could not be contacted at the time and the error message assumes too much. OTOH, the server should be up by that time (given ensureReady()).

All in all, this seems to work.

Please note the peculiar syntax for setting a plain String recipient. This is because XD DSL has special treatment for String literals and strips one layer of quotes.