Enhanced escape sequences

These commits change a few things about escape sequences:

* rename "set_tab_name" to "rename_tab", to match the already existing user command
* add a tab index argument to "rename_tab", so that you can rename any tab and not only the current one
* add a "new_tab" escape sequence to create a new tab
* add a "change_tab" escape sequence to switch to another tab

With all of this, it is possible to write quick'n'dirty scripts to setup your work environment:

```sh
printf "\033]777;tabbedex;new_tab\007"
sleep .05
printf "\033]777;tabbedex;new_tab\007"
sleep .05
printf "\033]777;tabbedex;new_tab\007"
sleep .05

printf "\033]777;tabbedex;rename_tab;;tmp\007"
printf "\033]777;tabbedex;rename_tab;1;build\007"
printf "\033]777;tabbedex;rename_tab;2;tests\007"
printf "\033]777;tabbedex;rename_tab;3;logs\007"

printf "\033]777;tabbedex;change_tab;1\007"
```