Feature: Disappear

[kettanaito]

What

I need to add a support for the tracker to recognize when the target element(s) disappears.

Why

• Visibility logic may rely on element disappearing in the viewport
• Once it is possible to recognize when element becomes invisible, the once logic may be enhanced to trigger multiple events after it becomes visible again, not on each successful snapshot (as now)

Caveats

• once should apply on disappearing logic as well
• How to handle disappearing callback? Should there be a dedicated snapshot just for disappearing even?