Integrate xdg_shell

[elinorbgr]

Behind a "nightly" feature, because xdg_shell still requires nightly

[elinorbgr]

All the more reason to do this, the feature blocking xdg_shell to nightly seems to be going to be stabilized: rust-lang/rust#29719 (comment)

[elinorbgr]

Oh wait, this is done!

Thanks again @mitchmindtree 😉