Allow to swap or add exercises in gym mode

[rolandgeider]

Somehow related to #15

The user should be able to swap exercises or add new ones on the fly

[rolandgeider]

Idea: add a new provider that controls the current state of the gym mode. This way it would be easier to e.g. swap exercises because a machine is currently occupied. just change the order or configure other things on the fly. This would probably also make it easier for us to do #179