Added an optional parameter to "CellToolbar.register_callback".

It lists the cell types the UI element must inserted in.

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

@@ -118,7 +118,9 b' var IPython = (function (IPython) {'
      * @method register_callback
      * @param name {String} name to use to refer to the callback. It is advised to use a prefix with the name
      * for easier sorting and avoid collision
-     * @param  callback {function(div, cell)} callback that will be called to generate the ui element
+     * @param callback {function(div, cell)} callback that will be called to generate the ui element
+     * @param [cell_types] {List of String|undefined} optional list of cell types. If present the UI element
+     * will be added only to cells of types in the list.
      *
      *
      * The callback will receive the following element :
@@ -154,9 +156,9 b' var IPython = (function (IPython) {'
      *      // user the ability to use it later
      *      CellToolbar.register_callback('foo', toggle);
      */
-    CellToolbar.register_callback = function(name, callback){
+    CellToolbar.register_callback = function(name, callback, cell_types) {
         // Overwrite if it already exists.
-        CellToolbar._callback_dict[name] = callback;
+        CellToolbar._callback_dict[name] = cell_types ? {callback: callback, cell_types: cell_types} : callback;
     };
 
 
@@ -256,6 +258,11 b' var IPython = (function (IPython) {'
             var key = preset[i];
             var callback = callbacks[key];
             if (!callback) continue;
+
+            if (typeof callback === 'object') {
+                if (callback.cell_types.indexOf(this.cell.cell_type) === -1) continue;
+                callback = callback.callback;
+            }
             
             var local_div = $('<div/>').addClass('button_container');
             try {