Support for external effect nodes

Currently user can export nodes (Node View -> Selected -> Export) which saves the node in qen JSON format. To get these nodes visible into "Add node" dialog, qen needs to be copied into "nodes" directory. This is OK when developing QQEM nodes, but not very usable for users especially when nodes are now installed with the binary / into Qt install path. It should be easy for users to make their own custom nodes and share the nodes with others.

Implement UI to add custom directories which will be seeked for nodes. Maybe also support some environment variable for the path? See that nodes model can be refreshed while QQEM is running.