Implement activation handling flawed

[pascalzauberzeug]

While reviewing #190, I noticed that the state-handling of implements is neither coherent nor working correctly.

All implements know whether they are activated or not and that state can be changed by Implement.activate and Implement.deactivate. They are however not used by the base navigation class, but called by the navigations that extend each other, which leads to activate being called multiple times at different stages of the preparation process.

Additionally WeedingImplent tracks it's own state which should not be the case. Especially because it's never stopped after being started.

When updating the KPIs, implements should either check if they are active or not. Or maybe better not handle the KPIs at all and let the KPI-provider do that.

[Johannes-Thiel]

With the changes that are Introduced in #190 the interenal statetracker was removed and the KPI function was moved to the base class. The check if the Impement is working is against the active property.