Missing prevent from abandonment support

[Lednerb]

At the moment the plugin does not support the $preventFromAbandonment functionality:

    /**
     * Indicates whether Nova should prevent the user from leaving an unsaved form, losing their data.
     *
     * @var bool
     */
    public static $preventFormAbandonment = true;

Steps to reporduce:

• Enable the above setting in the resource with a containing Froala-Field
• Edit only the Froala content and try to leave the page
• Page leave is possible without the alert window

It would be useful if this functionality can be added.