Fix crash when changing the grandparent of an item with focus.

Fix crash when changing the grandparent of an item with focus.

When an item was reparented it's sub focus item was cleared and those
of it's children were cleared along with those in the tree the item was
removed from.  Since we now rely on the focus state of an unparented
tree to be correct we need to restore the sub focus items of the
unparented tree.

Change-Id: I236c512751b99092c7e9a39194f4680304bfacc5
Reviewed-by: Martin Jones <martin.jones@nokia.com>