Refresh should provide feedback to the user while it is processing

[adietish]

follows up on #595
relates to #617

Steps:

1. EXEC: pick "Refresh" in the context menu for the cluster node

Result:
Refresh takes quite some time to actually refresh the tree (up to 7s for me using the sandbox). There's nothing that shows the user that it is doing anything while it is progressing.

Expected result:
Refresh should provide visual feedback to the user while it is proceeding. Ex. the cluster node could display a label "Refreshing..." or show sandglass icon

I started tackling this in #595 but separated it since any solution didn't work as expected. The "Refreshing..." label was only showing far too. One reason for it was that the actual way to update a label would refresh the whole tree (which would cause an additional, lengthy refresh of the tree).