window: make 'Calendar' the default title

Shows it for empty search results.

data/ui/window.ui

@@ -179,6 +179,7 @@
         <property name="visible">True</property>
         <property name="can_focus">False</property>
         <property name="show_close_button">True</property>
+        <property name="title" translatable="yes">Calendar</property>
         <child>
           <object class="GtkBox" id="button_box">
             <property name="visible">True</property>

src/gcal-window.c

@@ -1091,7 +1091,7 @@ search_changed (GtkEditable *editable,
         }
       else
         {
-          gtk_header_bar_set_title (GTK_HEADER_BAR (priv->header_bar), "");
+          gtk_header_bar_set_title (GTK_HEADER_BAR (priv->header_bar), _("Calendar"));
         }
     }
 }