theme/panel: Slightly increase horizontal padding on activities button

The activities button doesn't have an icon, which makes its horizontal
padding appear a bit smaller than the padding of most other items in the
panel.

We're going to indicate hover and focus of items in the panel using a
pill-shaped background color, which means the padding of those items
will get more visible. So give the activities button a special treatment
and slightly increase its padding to make sure the new indicator will
look good.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1397>

data/theme/gnome-shell-sass/widgets/_panel.scss

@@ -77,6 +77,10 @@ $panel_height: 1.86em;
       color: lighten($fg_color, 10%);
       &:focus, &:hover, &:active { color: lighten($fg_color, 10%); }
     }
+
+    &#panelActivities {
+      -natural-hpadding: $base_padding * 3;
+    }
   }
 
   .panel-button {