Don't change context if focus widget parent hierarchy doesn't define one

We now keep the current context if the focus is set on a widget with a
parent hierarchy that doesn't define one. This way, toplevels without a
context, like popup windows without parent, will keep the behavior from
before other toplevels could define contexts at all
(2118d542)

Task-number: QTCREATORBUG-8667
Change-Id: I8dd04d1f97f6ced10f8fa4269bc91457211396ca
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>