implements Processing providers
Plugins which implement providers should include the
hasProcessingProvider=yes
line within their metadata.txt file. This allows for rapid
identification of all plugins which implement Processing
functionality.
This command adds a plugin to active plugins and calls initProcessing(),
initializing only Processing related components of that plugin.
The new initProcessing() hook should be implemented by plugins
which provide Processing providers or algorithm, and should only
implement code which is required to load the provider and algorithms.
Strictly no GUI related code should be used here, that MUST
be moved out of initializers and deferred to the plugin's
initGui implementation.