Dapps interface RPC (#3311)

* Dapps Interface RPC

* Adding JS apis

* Support for signer interface in proxypac and embeds

* Fixing tests

* fixing tests again

js/src/api/rpc/parity/parity.js

@@ -60,6 +60,11 @@ export default class Parity {
       .then(outNumber);
   }
 
+  dappsInterface () {
+    return this._transport
+      .execute('parity_dappsInterface');
+  }
+
   defaultExtraData () {
     return this._transport
       .execute('parity_defaultExtraData');