media-gfx/gimp: 2.10.18 version bump

media-gfx/gimp: 2.10.18 version bump

dependencies update:
media-libs/babl-0.1.74
media-libs/gegl-0.4.22

add new dependency for online updates check (enabled by default):
>=dev-libs/json-glib-1.2.6

The check-update is disabled but the dependency is still required to build
(seems is used by menu item Help - Check Update)

Python2 scripts/plugin support is removed here due to dev-python/pygtk removal
https://bugs.gentoo.org/706462

Closes: https://bugs.gentoo.org/711548

Signed-off-by: Sergey Torokhov <torokhov-s-a@yandex.ru>
Signed-off-by: Joonas Niilola <juippis@gentoo.org>