gio: Add source tags to various GTasks constructed in GLib

This makes them easier to identify when debugging and profiling.

This patch was somewhat less than interesting to write.

https://bugzilla.gnome.org/show_bug.cgi?id=767765

gio/gfileicon.c

@@ -322,6 +322,7 @@ g_file_icon_load_async (GLoadableIcon       *icon,
   GTask *task;
 
   task = g_task_new (icon, cancellable, callback, user_data);
+  g_task_set_source_tag (task, g_file_icon_load_async);
   
   g_file_read_async (file_icon->file, 0,
                      cancellable,