This was a regression bug owing to a big qdoc cleanup
for Qt5. But the way QML inheritance had been handled
was not a good design, so it has been changed here.
When a .qml file is parsed by qdoc, the base type of
the QML component is detected, and its name is stored
in qdoc's tree node for the component. After qdoc has
parsed all the QML files, it traverses the tree, and
for each QML component that has a base type name but
no base type node pointer yet, it searches the tree
for the base type node and stores the pointer to
the node in the node for the components. Then when
the output generator generates the doc page for the
component, it has access to all the inherited members
in the base type.

Task-number: QTBUG-29569
Change-Id: Ib4958d05f55fa48a572f8ca51ffd57712f29bbc7
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>