Custom data sources for themes

[mathoudebine]

Is your feature request related to a problem? If so, please describe the problem.
I want to easily add custom data sources to this project (unsupported hardware sensors, sound from music players, web API data...)

Describe the feature / solution to your problem you'd like
An "custom" sensors class with empty methods to implement Python code to fetch my custom data. These custom sources can then be added to any themes easily.

Describe alternatives you've considered / and or tested
Change the code myself but it is difficult to hack into existing sensor classes

[mathoudebine]

Note: development has started in branch