Allow user update slice title in visualize flow (#3466)

* 1. after user make sql query and visualize, allow user click title to update slice title, and create a new slice at the same time.
2. don't save new title if it is empty. Will still show old title.

* change saveSlice call response and update explore view

superset/assets/javascripts/explore/main.css

@@ -104,3 +104,6 @@
   right: 0;
   top: 12px;
 }
+.save-modal-selector {
+  margin: 10px 0;
+}