[DRAFT] Add CMake support for XML documentation

[dsnopek]

I forgot to add CMake support in PR https://github.com/godotengine/godot-cpp/pull/1374

So far this PR is just refactoring the scons code so that CMake can generate the code via the doc_source_generator.py script. However, I haven't yet figured out how to add that to CMake yet. If anyone who knows CMake can help out, that'd be great! Otherwise, I will work something eventually. :-)

[Naros]

Hi @dsnopek so this is what I did to hack this for cmake:

1. Defined a simple godot-docs-generator.cmake file with this content. The paths could be arguments passed into the function from the main CMAKE file to allow maximum configuration, I just didn't go that far due to this being a PoC.

   FUNCTION( GENERATE_GODOT_DOCUMENTATION )
   # Grab all documentation XML files
   FILE(GLOB XML_FILES "${CMAKE_CURRENT_SOURCE_DIR}/doc_classes/*.xml")
   STRING(JOIN "," XML_FILES_STR ${XML_FILES})
   # Generate the target file
   SET(DOC_DATA_CPP_FILE "${CMAKE_BINARY_DIR}/_generated/doc_data.cpp")
   STRING(JOIN "," DOC_DATA_CPP_STR ${DOC_DATA_CPP_FILE})
   # Run python to generate the doc_data.cpp file
   EXECUTE_PROCESS(
           COMMAND cmd /c py ${CMAKE_CURRENT_SOURCE_DIR}/cmake/generate_godot_docs.py ${DOC_DATA_CPP_STR} ${XML_FILES_STR}
           WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
   )
   ENDFUNCTION()

2. In the generate_godot_docs.py, it contained your make_doc_source function verbatim, with this added to the bottom. I'm not a huge python guy, so might be a better way to do it:

   
   def main():
   if len(sys.argv) > 2:
       target_str = sys.argv[1]
       target_list = target_str.split(",")
       source_str = sys.argv[2]
       source_list = source_str.split(",")
       make_doc_source(target_list, source_list)

if name == "main": main()

3. In the main cmake, include the `generate-godot-documentation.cmake` and call the function, i.e.:
```cmake
INCLUDE(godot-docs-generator)
GENERATE_GODOT_DOCUMENTATION()

Now CMAKE will use python to run the script and generate the ${CMAKE_BINARY_DIR}/_generated/doc_data.cpp file. All the CMAKE then needs to do is to add that generated file to the list of sources, i.e.:

# Library sources
FILE(GLOB_RECURSE gdext_sources
        CONFIGURE_DEPENDS
        "${CMAKE_CURRENT_SOURCE_DIR}/src/*.[hc]"
        "${CMAKE_CURRENT_SOURCE_DIR}/src/*.[hc]pp"
        # Includes the generated doc data from /doc_classes
        "${CMAKE_BINARY_DIR}/_generated/*.cpp"
)

And that's it. It should be pretty straight forward to augment this for the current godot-cpp cmake, but likely with a few extra things since I did this in my downstream extension cmake.