Add index context property to QDeclarativeGeoMapItemBase

... for indexing alternative routes in a delegate
of QDeclarativeGeoMapItemView

Change-Id: If40c40bcc4e89c92c473b98fddaad163c73c08c3
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>