Gtk-WARNING Content added to the action area of a dialog using header bars

[Cubic PPA]

The warning, "Gtk-WARNING ** Content added to the action area of a dialog using header bars" occurs when running Cubic in Ubuntu 18.04.

[Cubic PPA (cubic-wizard)]

Reference item # in Bug #1779675...

https://bugs.launchpad.net/cubic/+bug/1779675/comments/1

Cubic does not use header bars, but the warning implies that it does.

This is not a critical issue, and seems to occur with GTK 3.22.

$ dpkg -l gir1.2-gtk-3.0
  Name Version
  ========================-=================
  gir1.2-gtk-3.0:amd64 3.22.30-1ubuntu1

[Cubic PPA (cubic-wizard)]

Fix available in:
- Cubic release version 2018.09-39
- Cubic trunk version 2018.09-173

Re-wrote the user interface for both GTK 3.10 and GTK 3.20.

- For GTK >= 3.20 improved responsive layout for all pages. Used CSD (Client
  Side Decoration) header-bars.

- For GTK < 3.20 improved responsive layout for all pages. Reused the GTK
  3.20 ui; used action-area for file-choosers and used GtkGrid for the
  footer-bar.