Add 'OpenShift: Set Active Project' command to change project visible in Application Explorer view

[dgolovin]

App Explorer view now only show resources for active project and the only way to switch to new one is use context with different project or activate project in k8s Clusters view.

There should be a command that would let to do that directly. Command should:

1. be available from command palette
2. show only not active project to select from
3. refresh the view after new active project selected

[dgolovin]

cc: @jeffmaury @sbouchet.