qdoc was not including QML signals in the documentation
for a QML type, when the signals were inherited from a
QML type that had been marked abstract. This is now fixed.

Additionally, the QML type qualifier is not shown in the
header for the documentation for a member of a QML type.
The type qualifier was redundant information, and it was
incorrect for members inherited from abstract base types.

Task-number: QTBUG-30111
Change-Id: I73aef40f69a15bca6948bf36596b83800d248b1f
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>