Support Accessible.ignored on non-leaf items

Ignoring items with children will make the children appear as children of
the parent of the ignored item.
Since setAccessibleFlagAndListener now only sets the flag we also rename
the function to just setAccessible

Change-Id: I79fc311509a3e454b4698274c63ad0e879fb93e3
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>