Rework handling of connection failures, to fix crashes.

Testcase: unplugging my network cable, waiting for timeouts, plugging it
back again later, check mail.

* Port from deprecated connectionLost to stateChanged, which has the major
benefit of not confusing "failure to connect" with "we lost our connection"
(due to KIMAP::Session emitting both signals for compat reasons).

* Failure to connect is now handled in the result slot for the LoginJob,
rather than via the connectionFailed signal, since the first one is called
anyway.

* Make sure we emit connectDone on error, even if m_initialConnectDone is true,
so that the resource sets itself to offline (rather than it attempting the
same task over and over again!).

* Don't set the resource status to Broken when it's simply offline
(login failure due to "could not connect" is handled separately now)

* Make sure not to pop up a password dialog when couldn't connect; this
is different from "we could connect but the server refused login".

Requires kdepimlibs commit acdb0f7e50 for ERR_COULD_NOT_CONNECT.

FIXED-IN: 4.8

(cherry picked from commit 757f464e5de7f992d0bfcb90dc97d4ea40df47d5)