Separate role-specific methods into their own traits

[jgcodes2020]

This PR separates Window into two traits: Surface and Toplevel (subtrait of Surface). Surface handles drawing- and event-related operations, while Toplevel handles things that only work for toplevel surfaces (maximizing/minimizing, etc.).

This architecture allows us to create other subtraits of Surface to represent specific window roles. I have provided the Subview trait as a potential example of what this could be used for.

[kchibisov]

Thanks.

But we generally still discussing how to approach stuff like that, because each platform has its own quirks.

I initially thought that the creation should be done from the parent (yes, which means not allowing to create with parent subsurface, but I think it doesn't make much sense anyway).

The end storage should be on the parent as well. There's also an issue how the addressing for stuff like that should work, in terms of IDs, etc and the fact that input events are delivered generally differently.

I'll ping you once we proceed with the winit-next, since this repo is more of a playground right now.