Save closed tabs to a "killed tabs" queue, for up to a max of "max-killed-tabs", and allow to re-open them. Keep the tab history but not its content. This integrates nicely with the session file, just add the "killed" flag as alias for TAB_KILLED and put it in the ktabshead queue upon initalization. It's thus possible to re-open closed tabs even after telescope has been closed, and tabs keeps their history! Add a new cmd, cmd_tab_undo_close, to try to re-open the most recently closed tab and bind it to `u'. While here, add a TODO for the tab history memory leak.

"set set-title = 1" is quite a mouthful. The old name will still be supported for a while to ease backwards compatibility.

Suggested by Florian, thanks!

If tab-bar-show is -1 hide the tab bar permanently, if it's 0 show it unconditionally. If it's 1, show the bar only when there is more than one tab.

This is achieved by calling `autosave_hook' in interesting places, like new_tab or free_tab. It'll set up a timer to later persist the session. This is particularly useful to avoid loosing tabs on the event of a crash or unexpected system halt, or other similar event.