Introduce global timed handlers

There are situations when applications need more predictable timed
handlers that don't depend on connection status. Other usecase of
the global handlers is to manage offline connections, for example,
reconnect after a disconnection.

strophe.h

@@ -279,7 +279,7 @@ void xmpp_send_raw(xmpp_conn_t *const conn,
 
 /* handlers */
 
-/* if the handle returns false it is removed */
+/* if the handler returns false it is removed */
 typedef int (*xmpp_timed_handler)(xmpp_conn_t *const conn,
                                   void *const userdata);
 
@@ -290,6 +290,17 @@ void xmpp_timed_handler_add(xmpp_conn_t *const conn,
 void xmpp_timed_handler_delete(xmpp_conn_t *const conn,
                                xmpp_timed_handler handler);
 
+/* if the handler returns false it is removed */
+typedef int (*xmpp_global_timed_handler)(xmpp_ctx_t *const ctx,
+                                         void *const userdata);
+
+void xmpp_global_timed_handler_add(xmpp_ctx_t *const ctx,
+                                   xmpp_global_timed_handler handler,
+                                   const unsigned long period,
+                                   void *const userdata);
+void xmpp_global_timed_handler_delete(xmpp_ctx_t *const ctx,
+                                      xmpp_global_timed_handler handler);
+
 /* if the handler returns false it is removed */
 typedef int (*xmpp_handler)(xmpp_conn_t *const conn,
                             xmpp_stanza_t *const stanza,