Move action handlers to the Accessible attached object

With this change, instead of writing:

    function accessiblePressAction() { submit() }

You should write:

    Accessible.onPressAction: { submit() }

For the moment, only 4 actions are added:
press, toggle, increase and decrease.

The old style action handlers are deprecated, and removed from the
documentation. New style action handlers will be preferred in case an
item declares both styles.

Change-Id: I11919e631d8476d55540f94252757b911c44ade4
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>