Add tree Widget into VTKWeb

Change-Id: Iff41a996039d6b1340b5dcd72760c8f95fcc95b1
Merge topic 'vtkweb-tree-widget' into master

38676bdf Add tree Widget into VTKWeb
Merge branch 'master' of git://vtk.org/VTK into delaunay-2d-desired-output-precision

Change-Id: I70701b9d964ceaac0edda01cd80144be8c77abaa