Treat parentless items as focus scopes

The root item of a tree is implicitly a focus scope simply because it is
the root of the tree. QQuickRootItem could gain the focus scope flag in
order to solve this for most cases, but there would still be a
possiblity of a crash for disconnected trees.

Change-Id: I6e04f11df4268fb3b96660d50707d70935a5dc5e
Reviewed-by: Martin Jones <martin.jones@nokia.com>