When setting tab_bar_switch_to_previous_when_closing_current_tab kitty
will now switch to the left tab instead of the previously active one
when the currently active tab is closed. This makes the closing of tabs
a bit more predictable.
Note that we are not touching the handling of the active_tab_history at
all. I was considering it, but we want to keep track of it in any case
to keep the 'switch to previous tab' shortcut working.
