Add title placeholder text

src/iridium-window.ui

@@ -75,7 +75,7 @@
                 <child>
                   <object class="GtkEntry" id="title_entry">
                     <property name="visible">True</property>
-                    <property name="text">Foo</property>
+                    <property name="placeholder-text">Add title …</property>
                     <property name="margin-top">6</property>
                     <property name="margin-left">6</property>
                     <property name="margin-right">6</property>