LibGUI: Shrink GUI::Splitter by 1 pixel to make it look just right

Splitters were slightly oversized to work around the fact that we
were ignoring 2px on both sides of them. Now that the whole splitter
can be interacted with, we can lose 1px of fat and look great! :^)

Libraries/LibGUI/Splitter.cpp

@@ -38,7 +38,7 @@ Splitter::Splitter(Orientation orientation)
     set_background_role(ColorRole::Button);
     set_layout<BoxLayout>(orientation);
     set_fill_with_background_color(true);
-    layout()->set_spacing(4);
+    layout()->set_spacing(3);
 }
 
 Splitter::~Splitter()