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.