core: add pointer in some callbacks (closes #406)

This pointer is the first argument received by callbacks, and the
existing argument "data" is now automatically freed by WeeChat when the
object containing the callback is removed.

With this new pointer, the linked list of callbacks in scripts has been
removed. This will improve speed of scripts (using a lot of hooks),
reduce memory used by scripts and reduce time to unload scripts.

Following functions are affected in the C API:

* exec_on_files
* config_new
* config_new_section
* config_new_option
* hook_command
* hook_command_run
* hook_timer
* hook_fd
* hook_process
* hook_process_hashtable
* hook_connect
* hook_print
* hook_signal
* hook_hsignal
* hook_config
* hook_completion
* hook_modifier
* hook_info
* hook_info_hashtable
* hook_infolist
* hook_hdata
* hook_focus
* unhook_all_plugin
* buffer_new
* bar_item_new
* upgrade_new
* upgrade_read

tests/tests.cpp

@@ -65,13 +65,14 @@ IMPORT_TEST_GROUP(Util);
  */
 
 int
-test_print_cb (void *data, struct t_gui_buffer *buffer,
+test_print_cb (const void *pointer, void *data, struct t_gui_buffer *buffer,
                time_t date, int tags_count,
                const char **tags, int displayed,
                int highlight, const char *prefix,
                const char *message)
 {
-    /* make C compiler happy */
+    /* make C++ compiler happy */
+    (void) pointer;
     (void) data;
     (void) buffer;
     (void) date;
@@ -105,6 +106,7 @@ test_gui_init ()
                 NULL,  /* message */
                 1,     /* strip colors */
                 &test_print_cb,
+                NULL,
                 NULL);
 
     /*