VTE: use proper accessor for the vertical adjustment

Use vte_terminal_get_adjustment() to get the adjustment, and provide an
appropriate default implementation for use if it isn't available (on
VTE < 0.9 or > 0.36).
VTE: stop using vte_terminal_set_font_from_string()

vte_terminal_set_font_from_string() has been removed in 0.38, so use
vte_terminal_set_font() instead.
VTE: drop support for selecting the emulated terminal

VTE 0.38 dropped the capability to emulate various terminals to only
support xterm.  This shouldn't be so much of a problem for us, as our
setting to select the emulated terminal is hidden since 2008 and
defaults to xterm since forever, and xterm is highly compatible.
VTE: add support for vte_terminal_spawn_sync()

VTE 0.38 removed vte_terminal_fork_command() in favor of the new
vte_terminal_spawn_sync(), so add support for this.
VTE: add debugging info to known which library was actually loaded
VTE: don't hard-require vte_terminal_set_word_chars()

VTE 0.38 removed it, so only use it when available.