This also fixes a couple of warnings at places (out of order init for
instance) and a potential bug in the webflow credentials / qtkeychain
integration.
Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>
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)
