Add missing impls for Atom

[Susurrus]

Fixes necessary for gtk-rs/gtk#478

[EPashkin]

👍 without it function returning Atom not work

[Susurrus]

As part of this Atom work we're also missing all of the Atoms specified on the Selections page. I don't even see them in gdk-sys though, so should I file a separate issue on the sys repo or how should I go about adding these?

[EPashkin]

May be it can be done as https://github.com/gtk-rs/gdk/blob/master/src/atom.rs#L11 but it need checked.
Also we don't have any of selection functions, so maybe these const need go to separate manual/selection.rs with adding some functions.
Also seems these selection functions X11 related and need be hidden under #[cfg(target_os = "linux")] but may be not, currently I don't ready fo check it.
Maybe @GuillaumeGomez know more about it?
I not against add constants somewhere if its really needed, but not sure if it need be added in this PR as it stall other PRs.

[Susurrus]

Right now the only thing that would be nice would be the constants. I'll add those as a separate issue and we can keep this PR focused on just this small part!

[GuillaumeGomez]

@EPashkin: If it's X11 specific, then we need to make conditional compilation for it like we did in some other parts as well.

[EPashkin]

@GuillaumeGomez X11 specific part moved to separate issue #149 (if it really specific)

[GuillaumeGomez]

Ah my bad. Then it's all good for me as well. Thanks @Susurrus!