tabswitcher: switch to next-in-line doc when top document is closed

At present, a typical scenario might be as follows. Use (e.g. lspclient) goto-definition in document A, then that may open a new document B. After having had a look, that document is no longer needed, so close document B. Then more likely than not, one will be in document C, a "seemingly random" document (based on internal structure order) rather than back in document A as would be more convenient based on tabswitcher's usage order.

More generally, closing the current document (= top document in tabswitcher order) should move to the next-in-line document. This patch makes tabswitcher monitor that scenario and adjust accordingly.