qmlplugindump: fix prototypes identification for composite types

The prototype name for the composite type is the first "default"
prototype found in the reachable types. If none is found then
the default value is QObject.

Change-Id: I40b4fefaab40e4c9f83d24c89bd026579be63e7b
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Doc: PathView::currentItem is an Item, not an int.

Change-Id: If06c747b424ac8dd6b880aa296af5e5957bb9b51
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>