Support store history of navigation

[parisni]

currently i3-back supports returning to last focused window. Let's say w-1. I need to be able to store the history so being able to jump to w-2 or w-n.

Currently you add a marks=_back. What about adding a second mark, let's say _back-$TIMESTAMP. That way this would be a basis to list all windows with the _back-$TIMESTAMP and order them by timestamp.

People would be able to then show them for eg in tools such roofy/woofy and chose the window in the history, sorted by last focus

WDYT