filediff.ui: Update dialog action area layout for new GTK+ (bgo#756390)

The now-default Adwaita styling looks really bad if you don't have your
dialog layout exactly what constitutes the new normal.

Author: kaiw

data/ui/filediff.ui

@@ -181,7 +181,6 @@
   </object>
   <object class="GtkMessageDialog" id="check_save_dialog">
     <property name="can_focus">False</property>
-    <property name="border_width">5</property>
     <property name="type_hint">dialog</property>
     <property name="skip_taskbar_hint">True</property>
     <property name="message_type">warning</property>
@@ -197,7 +196,7 @@
           <object class="GtkButtonBox" id="dialog-action_area1">
             <property name="visible">True</property>
             <property name="can_focus">False</property>
-            <property name="layout_style">end</property>
+            <property name="layout_style">expand</property>
             <child>
               <object class="GtkButton" id="close_button">
                 <property name="label" translatable="yes">Close _without Saving</property>
@@ -208,7 +207,7 @@
               </object>
               <packing>
                 <property name="expand">False</property>
-                <property name="fill">False</property>
+                <property name="fill">True</property>
                 <property name="position">0</property>
               </packing>
             </child>
@@ -222,7 +221,7 @@
               </object>
               <packing>
                 <property name="expand">False</property>
-                <property name="fill">False</property>
+                <property name="fill">True</property>
                 <property name="position">1</property>
               </packing>
             </child>
@@ -236,7 +235,7 @@
               </object>
               <packing>
                 <property name="expand">False</property>
-                <property name="fill">False</property>
+                <property name="fill">True</property>
                 <property name="position">2</property>
               </packing>
             </child>