Move VTK forward to support future use of crossbar.io.

This commit also includes a fix to pv_web_visualizer which sets a
reasonable default value to the dataDir member of _VisualizerServer.

Change-Id: I130ee5a46c532c827bdafef855391f30b999615a
Merge topic 'support-future-use-of-crossbar-io' into next

59214fcf Move VTK forward to support future use of crossbar.io.