Integrate KDE's decoration protocol

[elinorbgr]

An early version is available in wayland-protocols : https://docs.rs/wayland-protocols/0.20.1/wayland_protocols/misc/server_decoration/client/index.html

There is also ongoing discussion on the wayland mailling lists and some variant of the protocol is likely to be soon standardized : https://lists.freedesktop.org/archives/wayland-devel/2018-April/037933.html

The idea would be to make a ServerFrame<F: Frame> implementing window::Frame that would use the server decoration protocol if available and if not fallback to the F frame.

[elinorbgr]

Actually, it should probably not be a Frame instance, but rather an other method on Window to create it with the server decoration protocol support, as it requires access to a new global.

[emersion]

The xdg-decoration protocol has been merged in wayland-protocols 1.15.

[elinorbgr]

tentative implementation is up in #20, @emersion do you know of any compositor implementing xdg-toplevel-v1 for testing this? Does rootson already support it by chance?

[emersion]

We have a working implementation in this PR: swaywm/wlroots#1053

It's not yet merged because it requires a wayland-protocols update in the Arch repos (just bumped the maintainers though - hopefully we'll be able to merge soon).