Remove gtk+-3 build dep, install versioned gtk.css dirs unconditionally

Summary:
GTK will search /usr/share/themes/<theme>/gtk-<version> and then earlier
versions all the way back to 3.0.

See also:
https://developer.gnome.org/gtk3/stable/GtkCssProvider.html#GtkCssProvider.description

Test Plan: breeze-gtk installs gtk-3.x dirs to the expected location.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5853