Add keywords for dealing with stale elements

For my use case it would be nice to have some keywords to be able to deal with elements that get removed from the DOM. Something along the lines of

• Wait Until Element Is Stale
• Get Element Staleness
• Element Should (Not) Be Stale

I wrote a rudimentary Python plugin at work using Selenium's staleness_of condition. I got the OK to use the plugin as a base and spend some time so I'd be willing to try and provide a PR as well if this feature makes sense and would be a good fit for the library.

Inizia esaminando il plugin Python esistente e expected_conditions.staleness_of di Selenium, quindi confrontali con gli attuali pattern di keyword di SeleniumLibrary. Definisci le tre keyword proposte per gli elementi non più validi e verifica che coprano l'attesa, il recupero dello stato di obsolescenza e le asserzioni positive o negative.