Expose searchEdit accessible property to Qt Quick

Change-Id: I9230deaa75d1e9a9614f364d4adf92349c8a1901
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>
Remove logic that some accessible objects cannot have children

There are many places where this doesn't make sense (a button in a
search field, a line in a list represented as button with a child button
to remove the list item...).

Change-Id: Iab1d411adda696a01b1dcfdebb0a250066d6a7c2
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>