Revert "theme/panel: Remove spacing between Activities and appmenu"

As minor as the visual change is, it's still a UI change.

This reverts commit 91c4c43a7d.

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

@@ -25,6 +25,11 @@ $panel_transition_duration: 250ms; // same as the overview transition duration
     }
   }
 
+  // spacing between activities, app menu and such
+  #panelLeft, #panelCenter {
+    spacing: $base_spacing;
+  }
+
   // the rounded outset corners
   .panel-corner {
     -panel-corner-radius: $panel_corner_radius;