The proxyAuthenticationRequired() signal now goes to the
ProxyAuthHandler class. That class will try to read the proxy settings
from the keychain or ask the user about them.
We won't ask the user for credentials for explicitly configured proxies.
It also does not change how the credentials for explicitly configured
proxies are stored. (see #261)
The AccountManager does not belong in the libsync because it is not
part of the synchronisation algorithm, but is just an helper class
for the UI to maintain the account and read/save the config
