port to vte 0.38.0

VTE is not making any attempt to maintain API stability. The following
changes to the API impacted Termite, despite it avoiding deprecated
functions:

* vte_pty_set_term removed (hopefully without an impact...)
* vte_terminal_get_child_exit_status replaced with new signal parameter
* beep signal -> bell signal
* vte_char_attributes -> VteCharAttributes
* sync suffix added to I/O functions along with some parameter changes
* rgba suffix removed from all color functions
* inner-border attribute replaced with GtkStyleContext padding
* vte_terminal_set_font_from_string removed, use Pango directly
* vte_terminal_is_word_char removed - is_word_char copied to Termite
* vte_terminal_set_word_chars removed - feature dropped
* vte_terminal_set_visible_bell removed - feature dropped

Support for text selection is still not supported upstream. The required
API is tiny and does not expose internal details. Despite the lack of a
compelling reason to leave it out like a backwards compatibility risk,
the patch has been left to rot on the bug tracker.

The vte_terminal_get_user_shell_with_fallback API was also removed,
although it was an internal API and Termite was only using it for
convenience. The functionality has been moved inside Termite.

Closes #187