LibGUI: Allow tweaking each edge of TabWidget's content margins

Instead of having a single uniform margin around the child content of
a TabWidget, use a GUI::Margins to allow individual per-edge margins.

Userland/Applications/Browser/BrowserWindow.gml

@@ -12,7 +12,7 @@
 
     @GUI::TabWidget {
         name: "tab_widget"
-        container_padding: 0
+        container_margins: [0, 0, 0, 0]
         uniform_tabs: true
         text_alignment: "CenterLeft"
     }