Composer extension

Created by: nniclausse

This PR adds a new mechanism to extend the composer with nodes from other layers dynamically using a dtkComposerExtension class that can be implemented in a plugin. This plugin will itself load the necessary plugins of it's own layer . This way we can have a generic dtkVisualProgramming application The old dtkComposerFactory class is replaced by several sub-classes of dtkComposerExtension that records the nodes. This way, we can even build a specialized composer app without all the default nodes ( distributed or control nodes for ex.) . The namespace dtkComposer::node provides a node factory that can be extended by layers, and a initialize method that records all the default nodes