Implement windows 8.1+ notifications

This splits notifications up into multiple backends
currently only libnotify on unix and win8 toasts.

The win8 backend was originally written by @leeter
though heavily modified.

src/common/plugin.h

@@ -163,6 +163,7 @@ struct _hexchat_plugin
 };
 #endif
 
+GModule *module_load (char *filename);
 char *plugin_load (session *sess, char *filename, char *arg);
 int plugin_reload (session *sess, char *name, int by_filename);
 void plugin_add (session *sess, char *filename, void *handle, void *init_func, void *deinit_func, char *arg, int fake);