Move IGT and all dependent components into own MITK extension

Summary:

Third-party dependency changes

OpenCV and OpenIGTLink have been moved into rIGT MITK-IGT.

New MITK extension features

mitkFunctionAddExternalProject

Added PREPEND argument to mitkFunctionAddExternalProject.

Adds the external project to the front of the global external projects
list instead of to the end.

Customization of dependencies of external dependencies

Allow customization of external project dependencies.

For example, to enable OpenCV features of ITK, an extension author can add the following content to their CMakeExternals/Customization/ITK.cmake file (given, that they also use the PREPEND feature mentioned above):

set(CUSTOM_DEPENDENCIES
  OpenCV
)

set(CUSTOM_CMAKE_ARGS
  -DModule_ITKVideoBridgeOpenCV:BOOL=ON
  -DOpenCV_DIR:PATH=${OpenCV_DIR}
)

Doxygen EXAMPLE_PATHS

Make Doxygen EXAMPLE_PATHS customizable for MITK extensions.

Any *.cmake file in an <EXTENSION_ROOT>/Documentation folder is included
before MITK's Documentation/CMakeLists.txt is processed.

Use list(APPEND MITK_DOXYGEN_ADDITIONAL_EXAMPLE_PATHS ...) in such a file
to add additional example paths for Doxygen.

Fixes T30152.

Test Plan: CI. MITK-IGT is currently available in rIGT MITK-IGT until it will be imported by Alfred into a GitHub repository.

Reviewers: O3 MITK Reviewer Group II!, O1 MITK Reviewer Group I!

Maniphest Tasks: T30152

Differential Revision: https://phabricator.mitk.org/D901