theme: Use insensitive theming for undecorated insensitive buttons

Menu items such as "paste" when no clipboard is available were always
painted as active. This is because the undecorated buttons do not use
the insensitive color anymore.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2919>

data/theme/gnome-shell-sass/_drawing.scss

@@ -247,6 +247,7 @@
     background-color: none;
     box-shadow: none;
     &:insensitive {
+      @include button(insensitive);
       background-color: transparent;
     }
   }