Auto merge of #20228 - paulrouget:ports_refactor, r=jdm

Ports refactoring

Depends on https://github.com/servo/servo/pull/20071 and https://github.com/servo/servo/pull/19895

This PR does 3 things:
1) merge all the embedder coordinates callbacks into one
2) hand the embedder messages directly to the embedder
3) split the embedder code in 2 files: window.rs and browser.rs

This is in preparation for tabs support in `ports/`. We want to separate the windowing logic (glutin stuff) and the browsing logic (tabs management, browsers state, etc). It's also easier to bypass the callbacks and directly handle events.

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