refactor task class

[dhebbeker]

To consider for further development:

• do not use (directly) <Arduino.h>, use an abstraction instead
• do not check state in start() and stop() this allows to restart an interval at any time
• define member function to get the current state
• define Duration as seconds which shall be defined in a separate file
• define the type for labels in a separate file
• add the concept of IDs
• use the same type for timestampStart and recordedDuration

Originally posted by @dhebbeker in https://github.com/Task-Tracker-Systems/Task-Tracker-Device/pull/69#pullrequestreview-1711895979