Register a tab visit only after 500ms.

To use @mrmr1993's example:

- `5J` visits all of the tabs between the current tab and the target tab.  Yet we never really visited them at all.
- Nevertheless, these register as recently-visited and appear near the top of the list in the "Tabs" vomnibar.

Here, we delay registering a tab as visited for ~~750ms~~ 500ms (a magic number, which - by great good furtune - I pulled out of thin air).

Fixes #1233.