Should only list current namespace/project

[adietish]

We currently list all namespaces/projects in the tree that's displayed in the tool window. This makes the user scroll a lot until she/he gets to the resources she/he wants to work with. A better solution would be to only list the current namespace/project and offer a context menu to switch it. Once this items is chosen the list of available namespace/project would be displayed. This was implemented for vscode-tekton in https://github.com/redhat-developer/vscode-tekton/pull/320

related to #162