Ability for Transitioner to navigate (without transitions) when app is not active (in focus).

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**Is your feature request related to a problem? Please describe.**
I would like to be able to have a transition between pages of my application that are triggered in code after a potentially long running task.
If I run my demo application and click on the Next or Back buttons and then activate another window (e.g. Visual Studio) while the background delay is being awaited, when the process completes no Transition occurs.
However, if the demo app remains the active window it transitions as expected.

**Describe the solution you'd like**
For the page navigation to happen, ideally with transitions if the windows is visible.
If the window is not visible then page navigation should occur, but maybe without any transitions (instant change to control z-order)

**Describe alternatives you've considered**
Not sure of any other alternatives other than rolling my own Navigation functionality.

**Additional context**
My simplified demo app can be found in my Izzy-Transitioner-Demo branch of my fork (see: https://github.com/thepinkmile/MaterialDesignInXamlToolkit/tree/Izzy-Transitioner-Demo)

Commencez par le comportement de Transitioner décrit dans l’issue et reproduisez-le à l’aide de la branche Izzy-Transitioner-Demo liée, en comparant les fenêtres actives et inactives. Suivez la navigation déclenchée par Next/Back et définissez done comme l’achèvement de la navigation lorsque la fenêtre est inactive, en conservant les transitions lorsque la fenêtre est visible si cela est pris en charge.

Rédigé par le modèle d'indexation à partir du texte de l'issue.