Don't mark a space as translatable

There is no point in translating a string that consists just
of a single space. Pointed out in

https://bugzilla.gnome.org/show_bug.cgi?id=706778

gtk/gtkmessagedialog.ui

@@ -4,7 +4,7 @@
   <template class="GtkMessageDialog" parent="GtkDialog">
     <property name="can_focus">False</property>
     <property name="border_width">5</property>
-    <property name="title" translatable="yes"> </property>
+    <property name="title"> </property>
     <property name="resizable">False</property>
     <property name="type_hint">dialog</property>
     <property name="skip_taskbar_hint">True</property>