[Feature Request] Tab previews for unloaded tabs

[willhansen]

Description

Like many, I have a lot of tabs. A significant fraction of these are unloaded at any given time, to save RAM. It would be convenient to be able to preview their contents to aid in the "load, leave, or delete" triage process.

Of course, being unloaded, the page contents probably couldn't actually be directly displayed. I'm thinking the preview thumbnails could be saved immediately before unloading?