Implement message loop

[stuartmorgan-g]

The initial Win32 embedding (flutter/engine#9835) doesn't yet use the new message loop implementation that was recently added to the GLFW embedding, and uses a call that blocks on Windows events. That means this will regress to the earlier Windows behavior of not noticing plugin events unless there's an active native event stream, as described here.

[clarkezone]

I have a change going to resolve this.