Each ModifyDialogTabPage has its own instance,
	 * and it should add all relevant controls upon creation, always in the same sequence.
	 * This established a mapping of controls to indexes, which allows to restore the focus
	 * in a later session. 
	 * The index is saved in the dialog settings, and there is only one common preference for 
	 * all tab pages. It is always the currently active tab page which stores its focus
	 * index.
Each ModifyDialogTabPage has its own instance,
	 * and it should add all relevant controls upon creation, always in the same sequence.
	 * This established a mapping of controls to indexes, which allows to restore the focus
	 * in a later session. 
	 * The index is saved in the dialog settings, and there is only one common preference for 
	 * all tab pages. It is always the currently active tab page which stores its focus
	 * index. | ModifyDialogTabPage.java