Use widget's preferred size by default

[wmww]

The old behavior caused Flutter windows to have zero size and not show up unless they explicitly requested a size.

By opening this pull request, I agree for my modifications to be licensed under whatever licenses are indicated at the start of the files I modified

[wmww]

Hmm, looks like this breaks something to do with shrinking the window. In the demo once "fixed size" is toggled on toggling it off doesn't shrink the window back down to normal size.

[wmww]

This has sat for too long, and not going to risk stability for new features at this point.