Builder extensions with builder descriptors

* Adapted builder extensions base classes to work with builder descriptors only
* Reactivated extensions call from Builder app
* Removed no more used Simulation Listener extension type
* Adapted extension tests
Merge branch 'builder-ext-with-desc' of https://github.com/alibres/openfluid into alibres-builder-ext-with-desc
Observer to export spatial domain graph as .dot files

* Added export.spatial-graph.files.dot observer
* Removed dot export functionalities from PluggableFunction class
* Added tests

(closes #48)