Revert "Merge pull request #366 from wieslawsoltes/TabStripItem"

This reverts commit 46003152f4b5c5570d99e76edbed8012cb3ecc44, reversing
changes made to 44408cfc5caeaf22230d17ea2c4673d2f3841d87.

src/Perspex.Themes.Default/TabStrip.paml

@@ -15,6 +15,6 @@
     </Setter>
   </Style>
   <Style Selector="TabStrip > TabStripItem">
-    <Setter Property="Margin" Value="4"/>
+    <Setter Property="Margin" Value="16"/>
   </Style>
 </Styles>