Teach Examples to use ExternalData

Teach vtk_add_test_cxx and vtk_test_cxx_executable to work in
Examples/*/Cxx directories when variable "vtk-example" is set and
"vtk-module" is not set.  Convert Examples/*/Cxx/CMakeLists.txt to use
these functions.  Populate Examples/*/Cxx/Baseline/ directories with
ExternalData content links for example baseline images.

Change-Id: Ic7d847a4561f5ed882b35e80c3ce98aa578ef7d3
Merge topic 'use-ExternalData' into master

a731ca41 Teach Examples to use ExternalData