Add a popup form for the toolbarAdd button to create a new record

The form fields are automagically generated from the grid's `columns` object.
By setting the column property `required: true`, you can define fields as mandatory.
If a column is `editable`, the according field type and it's options are used, otherwise it defaults to `text`.
The form supports both local record adding (`grid.url = ''`) as well as remote adding, using the grid's url property.