changed dvd view

replaced textarea for actors with combobox, when adding or editing a dvd
(attention: only changed view, has to change logic of program to support
combo box)
added toString-method in model actor, to use object actor in combobox,
but only show the name of the actor
changed dvd view

changed logic to support combobox in actors;
added toString in model of director (to use object in combobox, but show
name);
changed view of dvd

replaced textfield of director with combo box
changed all function to support combo box
changed back function: when editing a dvd, back goes to show the dvd
without the changes
changed view of actors and dvd

changed back function: when editing a actor/director, back goes to show
the actor/director without the changes
automatically save function and new user navigation

when adding, editing or deleting a dvd, an actor or a director, the
system saves immmediately and automatically all the changes

when adding a dvd, an actor or director, the user sees the table with
all the dvds, actors or directors
interaction between dvd and actor/director

fill movie table of actor and director, when adding a new dvd
change movie table of actor and director, when editing or deleting a dvd
change actors or director of dvd, when editing an actor or a director
cannot delete actor or director, when actor or director is in dvd