Ryven Console official release and new inheritance model for widgets

Ryven Console is now "officially" supported. Once you created a flow in Ryven, if all nodes you used are compatible with Ryven Console (meaning they run without their GUI/widgets), you can then run the flow in Ryven Console, without any GUI. This way you can run intensive data processing tasks with much higher performance. Further changes:

• New inheritance system for widgets: Widget classes should derive from fixed base classes from now on (I updated existing nodes and the template files). This makes them safer and the source code files cleaner (as you don't have to implement all possible methods, they are pre-defined in the base classes).
• requirements: I added a requirements.txt file for all libraries to run Ryven and the example projects, so it's also convenient to set up a virtual environment now.