[SRU] Backport GTK transparent tab fixes

[Paul]

[Impact]

 * When using transparent tab background in two apps (guake and gnome-terminal), it results a broken tabbar which can be totally unusable.

 * Backport the patches to supported versions of Ubuntu will fix the user experience in those apps, and will polish the desktop look and feel.

[Test Plan]

 * Two apps got problems using transparent tabbar background, but the bug is simpler reproducible using "gnome-terminal":

   - Using Ubuntu 20.04, open "gnome-terminal";

   - Open app settings in hamburger menu;

   - Select your current profile in left sidebar;

   - Go to "Colors" tab;

   - Check "Use transparent background" checkbox, and select a transparent background using the related slider;

   - Close setting window;

   - Open a new tab using the button in the headerbar;

   - You should see a black background tabbar;

   - See related issue in Yaru repository: https://github.com/ubuntu/yaru/issues/2481

 * To reproduce the "guake" issue, you can see the related issue: https://github.com/ubuntu/yaru/issues/2149

 * The "guake" issue is more problematic because the full tabbar is transparent, so you can't read tabbar text.

[Where problems could occur]

 * All fixes strictly target those apps and no other, to avoid any side effect.

[Other Info]

 * The "gnome-terminal" issue is only related to Ubuntu 20.04, and a patch has been pushed to "ubuntu/focal" branch of the Yaru repository. The related PR is here: https://github.com/ubuntu/yaru/pull/2979

 * The "guake" issue affect all versions of Ubuntu. A patch has been pushed to "ubuntu/focal", "ubuntu/impish" and "master" branches of the Yaru repository. The related PR are here:

   - For the "master" branch: https://github.com/ubuntu/yaru/pull/3218
   - For the "ubuntu/impish" branch: https://github.com/ubuntu/yaru/pull/3219
   - For the "ubuntu/focal" branch: https://github.com/ubuntu/yaru/pull/3220

 * The patch add a fixed background to the tabbar of both apps.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.