python3-mb: get rid of exec()

[zzarne]

Concept draft:

• define an interface
• convert helpers.py to separate modules for each helper
• allow custom helper modules in ~/.jack_plugins

[zzarne]

Maybe we can get rid of the exec of the percent_fkt of helpers if we can replace it with a regex?

[pimzand]

To be honest, I never fully understood how the exec and fkt stuff works. I would be happy if you could replace it.