- Moved AssxmlExporter serialization code in a new file AssxmlFileWriter.cpp/h
- Added new files to CMakeLists.txt
- Replaced string format specifiers %i by %u to match argument type.
- Made a few changes in WriteDumb.cpp to call the new DumpSceneToAssxml function.
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, assimp team".
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, ASSIMP Development Team".
Changed copyright end year to 2020 in LICENCE.rtf.
Changed copyright end year in CMakeFiles.txt files and any other places referencing Assimp with a copyright start and end year.
