Doc: Updated content for Drill Down example

QtWidgets/Drill Down example uses images and information about Nokia's
old office locations as content. This change updates the example to
use concepts related to Qt instead.

In addition,
    - Documentation, screenshot updated accordingly
    - Corrected aspect ratio when scaling image items in the scene
    - Added a gray background with a slight gradient
    - Removed Symbian-specific code remnant

Task-number: QTBUG-31075
Change-Id: Id8abfbf7f4033f74172477570f8f28390854101c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>