QmlDesigner: Refactor NavigatorTreeModel::updateItemRowOrder

Change-Id: Idadaf9ff50b3d284ad64341a379db4ae1babbaa2
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: Make NavigatorTreeModel::InternalIdRole public

Change-Id: I348b49a180138908b6a16be6e63d7b5adf38a5da
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: Refactor NavigatorTreeModel::mimeData

Change-Id: Icafc4deb2e879acbbc32bb6950256f8b3494ac10
Reviewed-by: Marco Bubke <marco.bubke@digia.com>