Merge gtk320 and gtk318 into gtk3

Summary:
GTK 3.20 and GTK 3.18 are now merged into a single GTK 3.0 directory. This will reduce work needed in the future for maintenance and updates.

In the past, GTK3 had an extremely unstable theming API, however, as GTK4 draws near, less focus has been given to breaking existing CSS with GTK3. This makes serving specific versions of GTK CSS redundant and doubles the work required to make changes for gains most users aren't likely to see.

Test Plan: See that themes work properly.

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22876