Change dock panel spacing to 4px.

src/application/dockpanel.css

@@ -49,5 +49,5 @@
 
 
 #jp-main-dock-panel {
-  padding: 8px;
+  padding: 4px;
 }

src/application/shell.ts

@@ -112,7 +112,7 @@ class ApplicationShell extends Widget {
     rightHandler.stackedPanel.id = 'jp-right-stack';
 
     hboxPanel.spacing = 0;
-    dockPanel.spacing = 8;
+    dockPanel.spacing = 4;
     hsplitPanel.spacing = 1;
 
     hboxPanel.direction = 'left-to-right';