OIT Example: Don't shadow property with ID

Fix [id-shadows-member] qmllint warnings.

Drive-by edit: Make StartupView follow the documented recommendation for
View3D.camera, "It is strongly recommended to set this property
explicitly rather than relying on automatic camera selection"

Fixes: QTBUG-151397
Pick-to: 6.12
Change-Id: I51dafe173b4cd92a58560f77a4feec3027c13def
Reviewed-by: Antti Määttä <antti.maatta@qt.io>