Remove requirement to use marker interface

[mikechristiansenvae]

It would be nice if I wasn't required to use the IWpfShell marker interface to have the startup window. This requirement means that specific Window is special, and it is so at compile time. Using attributes has similar problems.

As an alternative, consider if there was an IStartupWindowFactory interface, whose duty was to provide a list of startup Window. It could provide different Window based on whatever factors the developer chooses.

The default implementation of IStartupWindowFactory could use the current IWpfShell marker interface, making it a drop-in replacement.

See this repository / diff showing one way it could be done.

• You can see that the IWpfShell interface has been removed from the window. (Source)
• A default implementation of IStartupWindowFactory is included that uses IWpfShell. Since it's using TryAddSingleton, it will be added if and only if the user has not already added their own implementation. (Source)
• This shows the default implementation of IStartupWindowFactory. With this implementation there is no change in behavior from current. (Source)
• This shows how the user would create a custom IStartupWindowFactory (Source)
• This shows how the user would add that custom IStartupWindowFactory. Since it's using AddSingleton, it's going to override the default factory that was added with TryAddSingleton. (Source)