Support to wlr_layer_shell

[SergioRibera]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

Yup, this is another PR trying to integrate layer_shell support to winit, personally I use this feature a lot and even considered making my own windowing system to support this feature, but that would be counterproductive to the ecosystem and I prefer to add this feature so the rest can enjoy it.

My main goal is to try to preserve as much as possible the interaction with the default API, to some extent I think it is a success but there are things that can not be avoided to change.

Clearly and as you will notice this PR is strongly inspired by #2832 I decided to make another one due to the low activity on the original author's github instead of contacting him and contributing to his fork.

Fixes #2582

Unresolved design questions:

• I would imagine Anchor, KeyboardInteractivity, and Layer should be reexported in winit::platform::wayland
• Additionally this also gives us the possibility to do other cool things in wayland like moving the window at will or getting the position of the window (here we probably have to debate whether storing the position is wise).

[kchibisov]

The response is the same as for other PRs, not unless backend split is done, which I hope should be soon.

[SergioRibera]

The response is the same as for other PRs, not unless backend split is done, which I hope should be soon.

Well, it seems completely make sense to me, is there any site where they are discussing this topic? I will leave the PR open for now and keep it updated with respect to the main winit branch.

[kchibisov]

Everything you need is present on github issues here and opened PRs. You can subscribe to #3433 .

[SergioRibera]

Perfect, thank you, I also joined the matrix server that you have in the README, I will be in contact to help you with whatever you need 🦀