Simplify VMTK Slicer extension build system

Rename extension into "SlicerExtension-VMTK"

Update build system based on current Slicer Superbuild template

Remove unneeded "LibraryModule" loadable module. Packaging the VMTK
python modules should be sufficient.

Add VMTK to CPACK_INSTALL_CMAKE_PROJECTS so that associated project
is installed.

Use "slicerMacroBuildScriptedModule" instead of the deprecated "ctkMacroCompilePythonScript"

Within VMTK external project:

* Checkout more recent version of VMTK

* Set install sub directory using Slicer_INSTALL_* variables