BUG #14490: added ability to do unix style installs on OsX.

Added a new advanced CMake variable: PARAVIEW_DO_UNIX_STYLE_INSTALLS,
which when ON, result in ParaView doing unix-style installs rather
than generating an App bundle on "make install".

Note that this results in a <INSTALL_PREFIX>/bin/paraview executable
rather than a paraview.app bundle.

Fixed Python path and plugin location mechanisms to work with this
new way of installing on OsX.

Change-Id: I721a1498f13902c8b8c890ce5b186960e50a2c7c
Merge topic '14490_unix_style_installs_on_osx' into next

d8b54af BUG #14490: added ability to do unix style installs on OsX.