Adwaita: Fix a padding problem in action bars

Padding adds up. A selector like "actionbar box" matches any box
inside an actionbar, so nested boxes accumulate more and more
padding. The intention clearly was to just give padding to the
topmost box that is the direct child of the actionbar's revealer.

https://bugzilla.gnome.org/show_bug.cgi?id=762036

gtk/theme/Adwaita/_common.scss

@@ -2817,7 +2817,7 @@ frame border,
   &:backdrop { border-color: $backdrop_borders_color; }
 }
 
-actionbar box {
+actionbar > revealer >  box {
   padding: 6px;
   border-width: 1px 0 0;
 }

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -3452,7 +3452,7 @@ frame border,
   .frame:backdrop {
     border-color: #1f2222; }
 
-actionbar box {
+actionbar > revealer > box {
   padding: 6px;
   border-width: 1px 0 0; }
 

gtk/theme/Adwaita/gtk-contained.css

@@ -3585,7 +3585,7 @@ frame border,
   .frame:backdrop {
     border-color: #a5a5a1; }
 
-actionbar box {
+actionbar > revealer > box {
   padding: 6px;
   border-width: 1px 0 0; }