Add new features to logger plugin (command /logger, log level, level by buffer, mask by buffer, ..), fix some bugs

New features:
- new command /logger
- log level, to log only some messages, according to importance (task #8592)
- level by buffer: custom level for some buffers (or group of buffers)
- log filename mask by buffer (or group of buffers)
- marker line is added after display of backlog
- add "delete" callback for config file sections
- add "mkdir_parents" function to plugin API
- remove old log options in IRC plugin

Bug fix:
- marker line is set only when user switches buffer (not when a plugin force
  switch, like IRC plugin does when opening server or channel buffer)
- backlog fixed (sometimes lines were not properly displayed)

src/core/wee-string.c

@@ -854,7 +854,7 @@ string_free_exploded (char **exploded_string)
  */
 
 char *
-string_build_with_exploded (char **exploded_string, const char *separator)
+string_build_with_exploded (const char **exploded_string, const char *separator)
 {
     int i, length, length_separator;
     char *result;