Backport PR #5605: Two cell toolbar fixes.

1. When a new notebook is loaded we should update the state of the ``select`` element listing the cell tollbar presets in the main toolbar, so that its value reflects the ``celltollbar`` metadata entry of the notebook.

2. When a new notebook without ``celltoolbar`` metadata entry is loaded we should make sure that the cell tollbars are hidden. In particular, that is needed when you are working to a notebook with a certain cell toolbar preset and then you revert the notebook to a state with no cell toolbars.

IPython/html/static/notebook/js/celltoolbar.js

@@ -184,6 +184,10 b' var IPython = (function (IPython) {'
     CellToolbar.register_preset = function(name, preset_list) {
         CellToolbar._presets[name] = preset_list;
         $([IPython.events]).trigger('preset_added.CellToolbar', {name: name});
+        // When "register_callback" is called by a custom extension, it may be executed after notebook is loaded.
+        // In that case, activate the preset if needed.
+        if (IPython.notebook && IPython.notebook.metadata && IPython.notebook.metadata.celltoolbar === name)
+            this.activate_preset(name);
     };
 
 
@@ -223,6 +227,8 b' var IPython = (function (IPython) {'
             CellToolbar._ui_controls_list = preset;
             CellToolbar.rebuild_all();
         }
+
+        $([IPython.events]).trigger('preset_activated.CellToolbar', {name: preset_name});
     };
 
 

IPython/html/static/notebook/js/maintoolbar.js

@@ -170,6 +170,11 b' var IPython = (function (IPython) {'
             var name = data.name;
             select.append($('<option/>').attr('value', name).text(name));
         });
+        // Update select value when a preset is activated.
+        $([IPython.events]).on('preset_activated.CellToolbar', function (event, data) {
+            if (select.val() !== data.name)
+                select.val(data.name);
+        });
     };
 
 

IPython/html/static/notebook/js/notebook.js

@@ -2106,6 +2106,8 b' var IPython = (function (IPython) {'
         if (this.metadata.celltoolbar) {
             IPython.CellToolbar.global_show();
             IPython.CellToolbar.activate_preset(this.metadata.celltoolbar);
+        } else {
+            IPython.CellToolbar.global_hide();
         }
 
         // now that we're fully loaded, it is safe to restore save functionality