Doc: Fix errors in Qt Sensors QML example

This change fixes the errors in example QML code by replacing the
overuse of anchors with Column/Row components, and cleaning up by
refactoring reusable code into a new QML type.

Task-number: QTBUG-31076
Change-Id: Iade6f98d7db716848c44d4066aa23064929b32f2
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>