Implements basic application logic

Closed Jørgen Nummedal Sveberg requested to merge hphjalta/prosjekt:8-connectClasses into main

added start function to App, moved gradeManager to java folder so it runs, needs to be fixed later properly

added controller and connected to fxml

Made student list and the grade lists into TableViews

Added Grade class to interface the TableViews

Added logic to see average of student score on list

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