Memory leak on replacing selection/focusModel

Initially detected/reported against selectionModel of ChoiceBox, similar issue for other controls with their respective selection/focusModels (for details, see comment below).

Following failing test case for ChoiceBox. Reason seems to be that ChoiceBoxSelectionModel registers strong listeners to both itemsProperty and the content of the items. Replacing both with weakListeners lets the test pass.

other controls with a selectionModel might have a similar issue

Failing test (as usual, test for leak c&p'ed from available tests ... ):

    private WeakReference<SingleSelectionModel<?>> weakSmRef;
    /**
     * Memory leak on replacing selectionModel.
     */
    @Test
    public void testReplaceSelectionModelMemory() {
        weakSmRef = new WeakReference<>(box.getSelectionModel());
        SingleSelectionModel<String> replacingSm = ChoiceBoxShim.get_ChoiceBoxSelectionModel(box);
        box.setSelectionModel(replacingSm);
        attemptGC(10);
        assertNull("selectionModel must be gc'ed", weakSmRef.get());
    }
    
    private void attemptGC(int n) {
        // Attempt gc n times
        for (int i = 0; i < n; i++) {
            System.gc();
            System.runFinalization();

            if (weakSmRef.get() == null) {
                break;
            }
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
               System.err.println("InterruptedException occurred during Thread.sleep()");
            }
        }
    }