Add clickable links in the mode-line

Only lightly tested. And I guess this only works really for graphical terminals.

```
* lisp/tracking.el (tracking--status-for-mode-line): Renamed from
`tracking-status'
(tracking-add-buffer)
(tracking-remove-buffer, tracking-next-buffer)
(tracking-remove-visible-buffers, tracking-add-buffer): : Use
`tracking--status-for-mode-line'
```