Add support for gtk3 theme variant feature.

Summary:
Gtk3 supports theme variant feature, which allows application to specify
a variant theme to use.

Test Plan:
Test with gtk3-demo -> Application Class, select prefer dark theme, also test
with GTK_THEME=Breeze:dark

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8149