Added constraint to minimum window size.

In fair play I should implement get_preferred_width/width_for_height and
friends methods on the views, but since I'm lazy and these days I doubt
someone using calendar will have less than 800x600 pixels.

src/gcal-window.c

@@ -487,6 +487,7 @@ gcal_window_constructed (GObject *object)
                     object);
 
   gtk_widget_show (embed);
+  gtk_widget_set_size_request (GTK_WIDGET (object), 800, 600);
 }
 
 static void