app-editors/gedit: EAPI-7, python_gen_cond_dep, fix python opt

app-editors/gedit: EAPI-7, python_gen_cond_dep, fix python opt

* Port to EAPI-7
* Use new python_gen_cond_dep for better python plugins and libpeas
  handling (should be best we can do now somewhat related with bug 612870)
* Fix python optimization to happen under meson. Python files that benefit
  from optimization are both in sitedir and in gedit plugins dir, so we
  call python_optimize twice (once to get the default logic for sitedir,
  so we don't need to figure out the correct path ourselves, and the second
  for the plugin files)
* Reorder deps to be in order of apperance in meson.build, instead of
  old gone configure.ac order
* Reorder src_configure to meson_options.txt order
* Drop questionable glib[dbus] USE dependency
* Drop unused eutils and multilib eclass inherits
* Drop GNOME2_LA_PUNT leftover from gnome2.eclass use (not used
  anymore and libtool not relevant with meson)
* Move gsettings-desktop-schemas and gvfs to RDEPEND-only - meson.build
  doesn't check for gsettings-desktop-schemas, and gvfs was always
  runtime-only.
* "use vala &&" in front of vala_src_prepare - vala.eclass does this
  too, so it doesn't change anything, but it's clearer to do it this
  way, and is the convention we've ended up with - just stuck with
  the eclass behaviour for now, unfortunately.
* Depend on higher gobject-introspection minimum, for meson support fixes.
* Add a comment on why src_test is a no-op for now.

Closes: https://bugs.gentoo.org/612870
Package-Manager: Portage-2.3.79, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>