GUACAMOLE-514: Merge parameter definition and translations for VNC "username" field.

guacamole-ext/src/main/resources/org/apache/guacamole/protocols/vnc.json

@@ -19,6 +19,10 @@
         {
             "name"  : "authentication",
             "fields" : [
+                {
+                    "name"  : "username",
+                    "type"  : "USERNAME"
+                },
                 {
                     "name"  : "password",
                     "type"  : "PASSWORD"