media-libs/gexiv2: fix deps, add py3.8, disable vala tool

media-libs/gexiv2: fix deps, add py3.8, disable vala tool

* Fix gexiv2 minimum dep per meson, re-sort in appearance order
* Fix python dep to be USE=python conditional; add missing pygobject
* Add python3_8 to PYTHON_COMPAT - just simple g-i overrides that
  should be OK with it, and appear to be on other distros
* Add missing gtk-doc docbook dep
* Disable a new vala requiring tool. This would really have to be
  conditional to USE=vala, possibly behind an extra USE flag. But
  it has questionable value and is not shipped by other distros
  either, so just unconditionally disable it for the time being

After this I can consider this appropriate for stabling

Bug: https://bugs.gentoo.org/711784
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Mart Raudsepp <leio@gentoo.org>