[javascript] How to add a custom button to the toolbar that calls a JavaScript function?

In case anybody is interested, I wrote a solution for this using Prototype. In order to get the button to appear correctly, I had to specify extraPlugins: 'ajaxsave' from inside the CKEDITOR.replace() method call.

Here is the plugin.js:

CKEDITOR.plugins.add('ajaxsave',
{
    init: function(editor)
    {
    var pluginName = 'ajaxsave';

    editor.addCommand( pluginName,
    {
        exec : function( editor )
        {
            new Ajax.Request('ajaxsave.php',
            {
                method:     "POST",
                parameters: { filename: 'index.html', editor: editor.getData() },
                onFailure:  function() { ThrowError("Error: The server has returned an unknown error"); },
                on0:        function() { ThrowError('Error: The server is not responding. Please try again.'); },
                onSuccess:  function(transport) {

                    var resp = transport.responseText;

                    //Successful processing by ckprocess.php should return simply 'OK'. 
                    if(resp == "OK") {
                        //This is a custom function I wrote to display messages. Nicer than alert() 
                        ShowPageMessage('Changes have been saved successfully!');
                    } else {
                        ShowPageMessage(resp,'10');
                    }
                }
            });
        },

        canUndo : true
    });

    editor.ui.addButton('ajaxsave',
    {
        label: 'Save',
        command: pluginName,
        className : 'cke_button_save'
    });
    }
});