port to vte 0.38.0

VTE is not making any attempt to maintain API stability. The following
changes to the API broke Termite:

* update the text selection patch to cope with internal VTE changes
* vte_pty_set_term removed (hopefully without impact)
* vte_terminal_get_user_shell_with_fallback removed, do it in termite
* vte_terminal_get_child_exit_status replaced with new signal parameter
* beep signal -> bell signal
* vte_terminal_pty_new -> vte_terminal_pty_new_sync
* vte_char_attributes -> VteCharAttributes
* sync suffix added to I/O functions and all APIs changed
* rgba suffix removed from all color functions
* inner-border attribute replaced with GtkStyleContext padding

Regressions (some will be fixed before landing in master):

* vte_terminal_set_word_chars removed, word_chars feature dropped
* vte_terminal_is_word_char removed, word movement stubbed out
* vte_terminal_set_font_from_string removed, not font configuration
* vte_terminal_set_visible_bell removed - need to use the attribute?