add client certificate support for https

A new text preference field "certificate" is added to store the certificate. As specified in stunnel, the certificate should be in PEM format, starting with a private key and immediately followed by the corresponding certificate.  Plain text certificate should not be included.

The content of such a PEM certificate is to be paste into the 'Certificate' field in the UI. The field will be enabled when proxyType "HTTPS" is selected. For HTTPS proxies which do not require a client certificate, the field may be leave empty.