EWS: Refactor server connection and password retrieval

In the old behaviour the resource would attempt to connect to the server
at resource construction time, which is not the way it should work. This
was a corner cut done in early development, when I was still a novice
and I wanted it to just work. As we know cutting corners tends to
backfire eventually...

The updated implementation connects to the server when the resource is
brought online.

The password request code also received refactoring. Instead of
synchronously waiting for KWallet it now performs the opening
asynchronously together with a timeout for safety.

A new unit test ensures that the code behaves as designed.