Show loading indicator on topology changes

[jpellizzari]

Implements #1804

When a user changes topologies via click on the topology filter, or by clicking 'SHOW IN ', the loading indicator will show after 1 second. It will be cleared by the first nodes delta data arriving via websockets.

[jpellizzari]

@simon Could you take a look por favor?

[fbarl]

Two comments about the UI:

• When you are in the graph view mode and you click on the topology that's already selected, the loader shows up on top of the graph that is already there. One solution here would be to simply move your added line(s) into the if-clause above, so that nodesLoaded is set to false only when the topology actually changes. As a side note, I'd also extract all this shared code between CLICK_SHOW_TOPOLOGY_FOR_NODE and CLICK_TOPOLOGY into a switchTopology helper, just because there's a lot of repeated logic.
• Switch to the table mode happens now every time I change back to a big topology (instead of just the first time), which I think can be slightly annoying. This is because of the condition in line 521 which I think we could be fixed easily if we also add a boolean nodesLoadedFirstTime or something like that to the global state... or perhaps you can find a more elegant solution.

[jpellizzari]

@fbarl Updated

[fbarl]

LGTM!