Port glutin-based browser to libsimpleservo

[jdm]

Once #22065 is fixed, we will have one port that directly uses the libservo APIs, and two ports that use libsimpleservo. Given how simple the actual Glutin-based browser is, and how much of the implementation is duplicated with libsimpleservo, it probably makes sense to build it on top of our simple embedding API instead.

This work will likely require extending the embedding API provided in libsimpleservo to accommodate features of the desktop app that are not yet exposed through the embedding API.

Glutin desktop source: https://github.com/servo/servo/tree/master/ports/glutin
[simpleservo API]: https://github.com/servo/servo/tree/master/ports/libsimpleservo/api

[jdm]

#21305 would be a prerequisite to this work.

[jdm]

I don't have a lot of information to provide here; using my work from #22065 as a model is probably a good place to start.