This is to provide an explicit option to not use the system proxy setting for IMAP connections, as described in T7430.
If the system proxy settings are set to use the environment variables http_proxy etc (for compatibility with non-KDE applications), then they are also used for IMAP connections. This is rarely required and will fail unless the proxy is transparent enough. See https://bugs.kde.org/show_bug.cgi?id=407685 and related bugs.
This change adds a IMAP account option to use the proxy for connection (the default is off). This is the core part of the change; the visible GUI part will follow in another diff.