vte: update 0.68.0 -> 0.70.1

Transition to gi-docgen from gtk-doc.

Drop vala tweaks: gobject-introspection already does
it correctly (with STAGING_LIBDIR included).

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>