All python 2 specific files now work in python 3, so move
them to the common web/Python module and remove the Python2 module.

Also fix a few uses of .has_key(), which has been
removed in python 3.

Backport python 3 compatible changes from the latest version
of Autobahn. We need to keep using this version while we use
WAMP and the autobahn router.

Address review comments.

Comment update-common.sh on how to avoid failing option check
on windows.

README.kitware.md describes the changes for vtk to the
source Twisted. Correct the cmake source path.

The expat library does not configure whether it was built statically
through its headers since it is installed both as a static and shared
library in certain setups. Since VTK only uses one, define XML_STATIC
for users of the library if necessary.

Fixes #17002.

Remove the old version of twisted as prep for
importing from the third-party repository.

Provide a helper function that import scripts can use when the
third-party project defines custom Git attributes in its top-level
`.gitattributes` file.  These must be commented out because Git
does not support custom attributes in a subdirectory where we
where the third-party project is about to be merged.

It is no longer necessary.

This allows expat to read XML files larger than 2 GB.

Fixes #15522.

Alignment used to work only for TextAsPath true. Now it works
for TextAsPath false as well.

This file should not be at this level. It was added as part of an HDF5
update, but HDF5 should get a more narrow attribute file instead.

This is similar to what is already done in ITK and SimpleITK.

The new vtkTargetLinkLibrariesWithDynamicLookup.cmake file is slightly modified copy from
ITK/CMake/itkTargetLinkLibrariesWithDynamicLookup.cmake
The explanation of what this patch tries to achieve is documented in this file.

A new argument is introduced, called OPTIONAL_PYTHON_LINK.
When used, the module will be optionally be linked against libpython.
In the module.cmake files, most vtkPython dependencies were moved to COMPILE_DEPENDS, so that libpython is not added to the target_link_libraries() call.

The vtkPython is explicitely linked against the python libraries, as this is a python executable.

Also, the find_package calls for the PythonLibs were made optional when possible.

The XDMF3 project was not updated, this will need to be done separately if weak linking is wished for that project.

Fixes: #16068

Remove the BUILD_SHARED_LIBS options for 3rd Party verdict library. The
variable being set was overwriting VTK's BUILD_SHARED_LIBS option because
it was being set to the same name.

Furthermore, although set, the variable is not used indeed anywhere in its
CMakeLists to build the library as static or shared.

The code was performing a null pointer check
using (void*)Data <= 0 instead of Data == NULL 
(or nullptr). This results in a compilation
error with clang trunk (future clang 4.0).

ES2 is causing headaches and code mess and is
getting old. Remove for 8.0 so we can focus on
more modern APIs. Most Apple and Adroid devices
from the past 4 years support ES3.

This is used to set the dllimport macros and it cannot assume that
vtkConfigure.h is already included.

The current dependency relationship in vtk is unclear and misleading.
This MR tries to rewrite them based on header files inclusion of headers
and source files in each module. Corner cases are considered and
modules are sorted in alphabetical order to facilitate future reference.
See details in my gitlab python based script project. In future we can continue
using this script to clean the VTK Dependency easily from
time to time.

There seems to be a missing case that leaves some macros
undefined. Attempting to fix.

Fix dashboard warnings due to older versions of gcc not recognizing the
suppressed warnings.

Adds a mechanism to disable warnings for diy2 instead of suppressing them.

(cherry picked from commit 2f239599)

(cherry picked from commit 18fdd54f)