Create off-canvas for the configurations of the gridstack children

Problem/Motivation

Create off-canvas for the configurations of the gridstack children

Remaining tasks

1. Define a new route for the form
2. Create a form with configuration fields
3. Textfield for CSS classes
4. Select styles from the option set
5. That's all for now, but probably it will be updated in future versions.
6. Attach ajaxified link to the gridstack container children.
7. Save data about classes into gridstack storage