private void BT_DeleteGrade_Click(object sender, RoutedEventArgs e)
        {
            if (this.DG_Grades.SelectedIndex >= 0)
            {
                Grade gradeToDelete = (Grade)this.DG_Grades.SelectedItem;
                SchoolGradesManager.RemoveGrade(selectedSubject, gradeToDelete);

                ShowSubject();
            }
        }
        private void BT_NewTrimester_Click(object sender, RoutedEventArgs e)
        {
            string msg = "Voulez vous vraiment commencer un nouveau trimèstre ? Les notes seront effacées.";

            if (System.Windows.Forms.MessageBox.Show(msg, "Agenda - Virtuel", System.Windows.Forms.MessageBoxButtons.YesNo, System.Windows.Forms.MessageBoxIcon.Question) == System.Windows.Forms.DialogResult.Yes)
            {
                SchoolGradesManager.NewTrimester(Global.userData.schoolGrades);
                ShowSubject();
            }
        }
        //Add grade
        private void BT_OK_Click(object sender, RoutedEventArgs e)
        {
            if (selectedSubject == null)
            {
                return;
            }

            float grade = (float)this.NUD_AddGrade_Value.Value;
            float outOf = (float)this.NUD_AddGrade_OutOf.Value;
            float coeff = (float)this.NUD_AddGrade_Coef.Value;

            SchoolGradesManager.AddGrade(selectedSubject, new Grade(grade, outOf, coeff));
            ShowSubject();

            this.Panel_AddGrade.Visibility = Visibility.Collapsed;

            this.NUD_AddGrade_Value.Value = 10;
            this.NUD_AddGrade_OutOf.Value = 20;
            this.NUD_AddGrade_Coef.Value  = 1;
        }