Replace ' with " in application.ui

It's more friendly to older versions of intltool and it is overall
a good practice.

Author: piotrdrag

data/ui/application.ui

@@ -44,29 +44,29 @@ Josef Vybíral</property>
       </object>
     </child>
   </object>
-  <menu id='app-menu'>
+  <menu id="app-menu">
     <section>
       <item>
-        <attribute name='label' translatable='yes'>_Preferences</attribute>
-        <attribute name='action'>app.preferences</attribute>
+        <attribute name="label" translatable="yes">_Preferences</attribute>
+        <attribute name="action">app.preferences</attribute>
       </item>
     </section>
     <section>
       <item>
-        <attribute name='label' translatable='yes'>_Help</attribute>
-        <attribute name='action'>app.help</attribute>
-        <attribute name='accel'>F1</attribute>
+        <attribute name="label" translatable="yes">_Help</attribute>
+        <attribute name="action">app.help</attribute>
+        <attribute name="accel">F1</attribute>
       </item>
       <item>
-        <attribute name='label' translatable='yes'>_About Meld</attribute>
-        <attribute name='action'>app.about</attribute>
+        <attribute name="label" translatable="yes">_About Meld</attribute>
+        <attribute name="action">app.about</attribute>
       </item>
     </section>
     <section>
       <item>
-        <attribute name='label' translatable='yes'>_Quit</attribute>
-        <attribute name='action'>app.quit</attribute>
-        <attribute name='accel'>&lt;Primary&gt;q</attribute>
+        <attribute name="label" translatable="yes">_Quit</attribute>
+        <attribute name="action">app.quit</attribute>
+        <attribute name="accel">&lt;Primary&gt;q</attribute>
       </item>
     </section>
   </menu>