[Live Inspector] Avoid multiple instances of Live Inspector

[Remi05]

Currently, you can launch Outlines multiple times and have multiple instances of the Live Inspector one on top of the other. This isn't a great experience, we should ensure single instancing for Live Inspector.

Relevant doc:

• https://stackoverflow.com/questions/19147/what-is-the-correct-way-to-create-a-single-instance-wpf-application
• https://docs.microsoft.com/en-us/windows/apps/windows-app-sdk/applifecycle/applifecycle-instancing