This function appends a search path to the list of paths where to
look for plugins.
A so-called "search path" actually consists of both a
module directory (where the shared libraries or language modules
lie) and a data directory (where the plugin data is).
The plugin will be able to use a correct data dir depending on
where it is installed, hence allowing to keep the plugin agnostic
when it comes to installation location: the same plugin can be
installed either in the system path or in the user's home directory,
without taking other special care than using
Plugin.infoGetDataDir when looking for its data files.
If data_dir is NULL, then it is set to the same value as
module_dir.
This function appends a search path to the list of paths where to look for plugins.
A so-called "search path" actually consists of both a module directory (where the shared libraries or language modules lie) and a data directory (where the plugin data is).
The plugin will be able to use a correct data dir depending on where it is installed, hence allowing to keep the plugin agnostic when it comes to installation location: the same plugin can be installed either in the system path or in the user's home directory, without taking other special care than using Plugin.infoGetDataDir when looking for its data files.
If data_dir is NULL, then it is set to the same value as module_dir.