ENH: Remove leaves from a vtkTree.

Add a new filter vtkRemoveTreeLeafFilter to remove
leaf nodes from a vtkTree.

Change-Id: I4fd6866d679c871396ff4814af1bde058a1334ba
ENH: New filter to extract subtrees.

Add a new filter (vtkExtractSelectedTree) to extract or
remove subtrees from a vtkTree.
The implementation is similar to vtkExtractSelectedGraph.

Change-Id: Ib31ed4b26634f37a53b44369cd32ac414f050a08
Merge topic 'TreeDropTip' into master

d2ea912b ENH: New filter to extract subtrees.
15a2c632 ENH: Remove leaves from a vtkTree.