[Graph explorer] Generate legend when Louvain button is pressed
Generates a new legend when the Louvain button is pressed.
There is currently a problem where the nodes in each clusters are updated but the cluster list themselves isn't, causing some clusters to be missing and others to be empty. This can be worked around by switching to a different sidebar tab (data for example) and then back to legend. If anyone has any ideas how to debug this, help is very welcome!
Closes #689 (closed)