core, memory, plugins, stdui, frontends: look for DNS SRV records when server is not specified:

- fix bug 3
    - modify "Connection" params:
          - rename "Server" to "Force server" and "Port" to "Force port"
          - set the default value to empty string for both

src/core/constants.py

@@ -40,6 +40,9 @@ class Const(object):
     NO_SECURITY_LIMIT = -1
     INDIVIDUAL = "individual"
     GENERAL = "general"
+    # Parameters related to connection
+    FORCE_SERVER_PARAM = "Force server"
+    FORCE_PORT_PARAM = "Force port"
     # Parameters related to encryption
     PROFILE_PASS_PATH = ('General', 'Password')
     MEMORY_CRYPTO_NAMESPACE = 'crypto'  # for the private persistent binary dict