QmlDesigner.FormEditor: Adding check to FormEditorItem::isContainer

An item is not considered a container if the default property is
a component.

Change-Id: I73453fb17d6c181cdd79cd6620307e5c30cac09d
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.Navigator: more checks for the reparenting button

We check for defaultPropertyIsComponent() and the metaInfo has to be valid.

Change-Id: I957c8440facec0ba54e527f2eda40926c5637b31
Reviewed-by: Marco Bubke <marco.bubke@digia.com>