Improve libservo API (WindowMethods and WindowEvent)

[paulrouget]

This issue is part of an effort to improve Servo's embedding story. See https://github.com/paulrouget/servoshell/projects/2

As of now, the embedder must implement the WindowMethods trait.

I would split these methods in 4 groups:

• asking the embedder to perform an action
  • prepare_for_composite
  • create_compositor_channel
  • present
• asking the embedder about the geometry of the viewport
  • scale_factor
  • framebuffer_size
  • size
  • client_window
• asking the embedder about capabilities
  • supports_clipboard
• events. Notifying the embedder of a change. Up to the embedder to honour them:
  • set_inner_size
  • set_position
  • set_fullscreen_state
  • set_page_title
  • set_page_url
  • status
  • load_start
  • load_end
  • load_error
  • head_parsed
  • set_cursor
  • set_favicon
  • handle_key

I think we can offer a better API:

• create_compositor_channel: this could be a clonable object passed to Browser::new()
• geometry related methods could be removed, and instead, the browser object would have a updateGeometry(AGeometryStruct) method. AGeometryStruct would include the scale factor, the inner size and the window position.
• capability methods would be regular callbacks
• present and prepare_for_composite, and all the other events as events. (browser.getEvents()).

As for browser.handle_events, it accepts WindowEvent. Not everything is a window event. We need a better semantic here. Maybe ServoCommand.

[paulrouget]

Superseded by #15934