port to vte 0.38.0

* update the text selection patch to cope with internal VTE changes
* beep signal renamed to bell
* vte_pty_set_term removed (hopefully without impact)
* vte_terminal_pty_new renamed to vte_terminal_pty_new_sync
* vte_terminal_get_user_shell_with_fallback removed, do it in termite
* vte_terminal_get_child_exit_status replaced with new signal parameter
* vte_terminal_select_none renamed to vte_terminal_deselect_all