set window_position for ACE,gajim_themes and manage_sounds windows

data/gui/advanced_configuration_window.ui

@@ -5,6 +5,7 @@
     <property name="can_focus">False</property>
     <property name="border_width">6</property>
     <property name="title" translatable="yes">Advanced Configuration Editor</property>
+    <property name="window_position">center-on-parent</property>
     <property name="role">ace</property>
     <property name="default_width">650</property>
     <property name="default_height">540</property>

data/gui/gajim_themes_window.ui

@@ -25,6 +25,7 @@
   <object class="GtkWindow" id="gajim_themes_window">
     <property name="border_width">12</property>
     <property name="title" translatable="yes">Gajim Themes Customization</property>
+    <property name="window_position">center-on-parent</property>
     <property name="type_hint">dialog</property>
     <child>
       <object class="GtkVBox" id="vbox97">

data/gui/manage_sounds_window.ui

@@ -7,6 +7,7 @@
     <property name="height_request">350</property>
     <property name="border_width">6</property>
     <property name="title" translatable="yes">Manage sounds</property>
+    <property name="window_position">center-on-parent</property>
     <property name="type_hint">dialog</property>
     <signal name="delete_event" handler="on_manage_sounds_window_delete_event"/>
     <child>