Fix shift-enter behvaior in nbagg backend

Commit 86ac6ce undid the changes from commit d527799 to change the behavior of shift-enter in Jupyter notebook, as discussed in matplotlib#1124, matplotlib#4758, and matplotlib#6752.

With the changes here, shift-enter now selects the next cell, executes it, and then selects the following cell.

lib/matplotlib/backends/web_backend/js/nbagg_mpl.js

@@ -167,11 +167,13 @@ mpl.figure.prototype._key_event_extra = function(event, name) {
     if (event.shiftKey && event.which == 13) {
         this.canvas_div.blur();
         event.shiftKey = false;
-        // Send a "J" for go to next cell
-        event.which = 74;
-        event.keyCode = 74;
-        manager.command_mode();
-        manager.handle_keydown(event);
+        // select the cell after this one
+        var index = IPython.notebook.find_cell_index(this.cell_info[0]);
+        IPython.notebook.select(index + 1);
+        // execute the cell and select the next cell
+        var cell = IPython.notebook.get_selected_cell();
+            cell.execute(true);
+        IPython.notebook.select(index + 2);
     }
 }