Re-emit events from, er, Event objects

We do create Events in more places, but this is probably the only
place that matters since the only event is 'decrypted' which won't
fire for, eg. events we send.

src/client.js

@@ -3241,6 +3241,9 @@ function _resolve(callback, defer, res) {
 function _PojoToMatrixEventMapper(client) {
     function mapper(plainOldJsObject) {
         const event = new MatrixEvent(plainOldJsObject);
+        reEmit(client, event, [
+            "Event.decrypted",
+        ]);
         if (event.isEncrypted()) {
             event.attemptDecryption(client._crypto);
         }