Context view: Allow several active contexts at the same time (we currently only allow 1, switching is fast though)

[adietish]

related to #162

You currently cannot watch 2+ contexts in parallel (which can be a problem though bcs too many watches eat memory, cpu etc.).

• Lens lists all of them (like we currently do). You can also have several clusters "active" at the same time. They dont list individual items like pods in the navigator though, only the cathegory "Pods". You can list the pods in a editor that is watching them come and go though. That's how they avoid too many watches.
  The drawback is though, that you only see ex. all the pods that belong to a deployment after opening the editor that lists all the deployments and then open properties for your deployment (2 clicks while we list them right in the tree).
  

• Jetbrains Kubernetes Plugin has a different approach. It requires the user to explicitly add contexts (we list all available ones, KUBECONFIG env var or main config file)
  

• We could also improve the user experience by improvements when switching contexts instead. Bookmarks, restoring previous expansion states, etc. would help a lot. This could be a simple was to improve the current limitation since switching in our plugin is fast.