when freeing a tab we have to release also its history.

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.

The session file is now overloaded with the history info, that means that upon startup telescope can repopulate each tab history! The price to pay is that one can't start an old telescope with a new session file, otherwise will get a lot of fancy '<' and '>' tabs...

it's cleaner to just toggle the lazy bit in switch_to_tab instead of pushing that complexity in load_url. While there also try to simplify the function and its documentation from unnecessary checks and extra verbosity. This also fixes a bug where Telescope ended up pushing the url on the history *twice* at startup.

~/.telescope/session was parsed in the main process before dropping in the sandbox: that's no good. This moves the initialization of the ui events later in ui_main_loop and makes so that the fs process, after entering the sandbox, parses and sends the content of the last session back to the main process; when it's done the ui gets loaded and telescope is back rocking. I'm plannig to re-use the struct session_tab during the save session operation soon.

When enabled with the -S (or --safe) flag, prevent telescope from writing files to the disk.

this helps to correctly manage the tab bar when tab-bar-show is 1.