Adwaita: Set border-spacing for entries

When our entries contain icons, they should not run
directly into the text, so put some spacing between
the children of entries.

gtk/theme/Adwaita/_common.scss

@@ -283,6 +283,7 @@ entry {
     padding-right: 8px;
     border: 1px solid;
     border-radius: $button_radius;
+    border-spacing: 6px;
     transition: all 200ms $ease-out-quad;
 
     @include entry(normal);