OIT Example: Fix URL comparison to correctly hide the back button

The "Back" button is supposed to be invisible in the Startup view, but
it currently remains visible at all times. This is because, after
calling `loader.setSource("StartupView.qml")`,
loader.source === "qrc:/StartupView.qml" !== "StartupView.qml"

This patch makes the binding for backButton.opacity/visible update as
expected.

Pick-to: 6.12
Change-Id: I7c1fa6deb9d7091f7102601cbdfe2881d52a5f60
Reviewed-by: Antti Määttä <antti.maatta@qt.io>