Split `gl` into `gl` and `gL`

Split `gl` into `gl` and `gL`

`gl` selects the most recent tab. Simple, right? But what if you open a
background tab? Which is the most recent tab now? Or if you open several
background tabs?

Previously, if you opened several background tabs and then pressed `gl`, the
last opened new background tab would be selected. That behavior just happened to
be.

In my opinion, it is more consistent if `gl` always selects the last _visited_
tab. An unread background tab has _not_ been visited (yet). This commit makes
this change.

If there is only one visited tab (but possibly several unread tabs), a
notification is shown telling that there is no most recent tab.

The `gL` command selects the oldest unread tab. This is useful when having
opened a bunch of background tabs. `gL` then lets you step them through in the
order you opened them. (If there are no unread tabs, a notification is shown.)

In other words, `gl` deals with _visited_ tabs only (from now on), while `gL`
deals with _unread_ tabs only.

The motivation for this commit came from piroor/treestyletab#874.