Previously, we'd try migrating from legacy settings if reading
the settings failed with an error. Now, we try again after a
couple of seconds and eventually give up.
This will help moving the SyncEngine construction in the constructor
and allow moving functionalities from Folder to SyncEngine or its
delegated objects.
This will be useful if we ever want to store account-level gui state.
I built this originally because I thought a paused account would be
this kind of state.
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
