Support deactivating finders with page visibility API

[skeggse]

When a user navigates away from the page (by visiting a different tab, and hiding the current tab), it may sometimes be useful to deactivate some or all finders. Finders query the DOM on a timer, and it's harder for browser developers justify resource optimization on pages that are actively using normal timers as well as event-driven hooks like MutationObservers. While the page may update in the background, some page-parser-tree use-cases do not require monitoring of the page while it's in the background. This may help improve resource consumption on resource-constrained devices.