GTK 2 dependency will be removed in CEF v70+

[cztomczak]

GTK2 support was removed from Chromium in rev https://crrev.com/287339e073 . In CEF it is planned to remove GTK dependency in Chromium instead of upgrading from GTK 2 to GTK 3.

See upstream CEF issue:
https://bitbucket.org/chromiumembedded/cef/issues/2014/linux-remove-gtk-dependency-from-chromium

Related issue to add support for GTK 3 in cefpython: #446 ("Support linking to GTK 3 when building CEF from sources and when building cefpython module").

See all related GTK issues:
gtk

Related patches by Joseph Kogut that removes GTK 2 dependency in cefpython: jakogut@f35475f and jakogut@f21917d .