HttpConnectionクラスにある通信設定の初期化などの処理をNetworkingクラスに移動

HttpConnectionクラスにある通信設定の初期化などの処理をNetworkingクラスに移動

移動したプロパティ等:

 * HttpConnection.DefaultTimeout -> Networking.DefaultTimeout
 * HttpConnection.proxyKind -> Networking.ProxyType
 * HttpConnection.proxy -> Networking.Proxy
 * HttpConnection.GlobalHttpClient -> Networking.Http
 * HttpConnection.WebProxyChanged -> Networking.WebProxyChanged
 * HttpConnection.InitializeConnection() -> Networking.Initialize()
 * HttpConnection.SetWebProxy() -> Networking.SetWebProxy()
 * HttpConnection.CreateHttpClient() -> Networking.CreateHttpClient()
 * MyCommon.GetUserAgentString() -> Networking.GetUserAgentString()