X11 clipboard support

[aweinstock314]

Relevant to issue #5376

Depends on pull request servo/rust-xlib#36

Also depends on an external repository https://github.com/aweinstock314/rust-x11-clipboard, should this be moved into servo's github namespace?

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4468

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[jdm]

$ ./mach test-tidy
components/script/textinput.rs:130: trailing whitespace

[jdm]

Why's the rust-xlib dependency needed in script?

[pcwalton]

r-

This is going to prevent sandboxing, since it accesses X from within the script task.

[jdm]

Reviewed on Critic!

[aweinstock314]

In response to https://critic.hoppipolla.co.uk/showcomment?chain=11501 :

It could be recreated on-demand, but that would involve creating/closing the X11 Display/window each time. I hadn't measured to see if that's a performance issue in practice, it just seemed like the sort of thing that ought to be cached.

[jdm]

Two small style nits left, then we'll need to wait on the rust-xlib PR.

[larsbergstrom]

Please rebase this PR and then reopen. It has been temporarily closed to avoid generating excessive work for GitHub's servers.