gmessages: Add g_warning_once()

In many places the pattern

    static gboolean warned_once = FALSE;
    if (!warned_once)
      {
        g_warning ("This and that");
        warned_once = TRUE;
      }

is used to not spam the same warning message over and over again. Add a
helper in glib for this, allowing the above statement to be changed to

    g_warning_once ("This and that");

docs/reference/glib/glib-sections.txt

@@ -1411,6 +1411,7 @@ g_log
 g_logv
 g_message
 g_warning
+g_warning_once
 g_critical
 g_error
 g_info