set border_width to 6 px

otherwise Width and Height fields in Fill -> Bitmap will
be directly at the tab page border.

Also 12 px was too large on the left.

Change-Id: I4ae17020333d61bf80e02ea29989fbb4693da591

cui/uiconfig/ui/areatabpage.ui

@@ -22,8 +22,7 @@
   <object class="GtkBox" id="AreaTabPage">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
-    <property name="margin_left">12</property>
-    <property name="margin_top">6</property>
+    <property name="border_width">6</property>
     <property name="spacing">6</property>
     <child>
       <object class="GtkFrame" id="Fill Frame">