ENH: support plugin building for paraview-5.7

Note
    For paraview-5.7 (and later?) plugins are built into their own subdirs.
    It also is not possible to suppress creation of static libraries in
    the process.

The current solution is still a bit ugly
  - install into a local "staged" location (within the build directory)
  - use rsync to relocate to the .so files to the plugin directory