Implement Page Visibility

[gterzian]

Spec: https://html.spec.whatwg.org/multipage/interaction.html#page-visibility

There are at least two places where we could run the update-the-visibility-state steps:

When the document loads:

servo/components/script/dom/document.rs

Line 695 in 10ec856

task!(fire_pageshow_event: move || {

Which would match: https://html.spec.whatwg.org/multipage/browsing-the-web.html#updating-the-document:update-the-visibility-state

And when the document unloads:

servo/components/script/dom/document.rs

Line 2221 in 10ec856

// TODO Step 6, document visibility steps.

which would match: https://html.spec.whatwg.org/multipage/document-lifecycle.html#unloading-documents:update-the-visibility-state