Replace WindowGroup with something simpler

[dblohm7]

WindowGroup supports creating windows across multiple threads in the same application (!). This is a really bad idea on Windows, and shouldn't even be supported IMHO.

We should yeet WindowGroup and replace it with a single-threaded equivalent. MessagePump or something.

Also a main window shouldn't be responsible for its creation; it should be the first thing created as part of an app, and then a main window created off of it.

Yes, these are breaking changes and we should re-version our fork of Walk accordingly.

[dblohm7]

I've been dealing with is as a pseudo-FixitFriday thing, even though it's much larger in scope than that.