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.

Beginne mit der Überprüfung des bestehenden Python-Plugins und von Selen现场ums expected_conditions.staleness_of und vergleiche sie anschließend mit den aktuellen Keyword-Mustern von SeleniumLibrary. Definiere die drei vorgeschlagenen Keywords für veraltete Elemente und verifiziere, dass sie das Warten, das Abrufen des Veraltetenstatus sowie positive oder negative Assertions abdecken.