add_search_path

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 [method@PluginInfo.get_data_dir] when looking for its data files.

If data_dir is null, then it is set to the same value as module_dir.

Parameters:

this	A Engine.
module_dir	the plugin module directory.
data_dir	the plugin data directory.