Is your feature request related to a problem? Please describe.
Currently Application is almost completely static methods and properties. This prevents defining alternative implementations and tests cannot Moq expectations of the static methods their members invoke.

Describe the solution you'd like

1. Change all Application methods to not be static (adding virtual where it makes sense)
2. Implement Singleton pattern i.e. everyone accesses Application.Instance which is an IApplication
3. Update all call sites

This will enable adding alternative implementations to IApplication e.g. #3837

To do this I could for example subclass Application but override certain methods, or implement IApplication from scratch. We would then change the instance in Program.cs (i.e. at start of program)

// This would throw unless the current instance is Shutdown or not yet Init
Application.ChangeInstance(myAltApplicationClass)

Describe alternatives you've considered
An alternative is to keep public API how it is and add the singleton pattern behind the static methods i.e. the static methods all forward to an internal singleton. This would be more code to write but would look prettier for the user.

Additional context
Add any other context or screenshots about the feature request here.