Add proxy config with Socks5 and HTTP proxy support

Resolve #547

This patch attempts to add proxy config support to NeoChat so people could set a HTTP or Socks5 proxy for anonymity or privacy/censorship-circumvention reason.

Currently this will only allows setting proxy type, host and port (which should be usable for most of the cases). Settings in that page needs to be applied by clicking the Apply button so the proxy setting won't accidentally get changed.

Proxy is disabled (use System Default option) by default.