preferences.ui: Update preferences dialog to new style with headerbar

Author: kaiw

data/ui/preferences.ui

@@ -86,64 +86,23 @@
     </data>
   </object>
   <object class="GtkDialog" id="preferencesdialog">
-    <property name="can_focus">False</property>
-    <property name="border_width">5</property>
+    <property name="border_width">0</property>
+    <property name="modal">True</property>
+    <property name="resizable">False</property>
     <property name="title" translatable="yes">Meld Preferences</property>
     <property name="type_hint">dialog</property>
+    <property name="use-header-bar">1</property>
     <signal name="response" handler="on_response" swapped="no"/>
     <child internal-child="vbox">
       <object class="GtkBox" id="dialog-vbox4">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
         <property name="orientation">vertical</property>
-        <child internal-child="action_area">
-          <object class="GtkButtonBox" id="dialog-action_area4">
-            <property name="visible">True</property>
-            <property name="can_focus">False</property>
-            <property name="layout_style">end</property>
-            <child>
-              <object class="GtkButton" id="helpbutton1">
-                <property name="label">gtk-help</property>
-                <property name="can_focus">True</property>
-                <property name="can_default">True</property>
-                <property name="receives_default">False</property>
-                <property name="use_stock">True</property>
-              </object>
-              <packing>
-                <property name="expand">False</property>
-                <property name="fill">False</property>
-                <property name="position">0</property>
-              </packing>
-            </child>
-            <child>
-              <object class="GtkButton" id="closebutton1">
-                <property name="label">gtk-close</property>
-                <property name="visible">True</property>
-                <property name="can_focus">True</property>
-                <property name="can_default">True</property>
-                <property name="receives_default">False</property>
-                <property name="use_stock">True</property>
-                <accelerator key="Escape" signal="clicked"/>
-              </object>
-              <packing>
-                <property name="expand">False</property>
-                <property name="fill">False</property>
-                <property name="position">1</property>
-              </packing>
-            </child>
-          </object>
-          <packing>
-            <property name="expand">False</property>
-            <property name="fill">True</property>
-            <property name="pack_type">end</property>
-            <property name="position">0</property>
-          </packing>
-        </child>
+        <property name="border-width">0</property>
         <child>
           <object class="GtkNotebook" id="notebook">
             <property name="visible">True</property>
             <property name="can_focus">False</property>
-            <property name="border_width">5</property>
             <property name="show_border">False</property>
             <child>
               <object class="GtkVBox" id="editor_tab">
@@ -1586,10 +1545,6 @@
         </child>
       </object>
     </child>
-    <action-widgets>
-      <action-widget response="-11">helpbutton1</action-widget>
-      <action-widget response="-7">closebutton1</action-widget>
-    </action-widgets>
   </object>
   <object class="GtkSizeGroup" id="sizegroup_editor">
     <widgets>