Use a shorter/simpler license convention, stops the header taking so
much space.
Follow the SPDX license specification: https://spdx.org/licenses
- C/C++/objc/objc++
- Python
- Shell Scripts
- CMake, GNUmakefile
While most of the source tree has been included
- `./extern/` was left out.
- `./intern/cycles` & `./intern/atomic` are also excluded because they
use different header conventions.
doc/license/SPDX-license-identifiers.txt has been added to list SPDX all
used identifiers.
See P2788 for the script that automated these edits.
Reviewed By: brecht, mont29, sergey
Ref D14069
Splits up `tree_element_id.cc`/`tree_element_id.hh`.
If we move all ID types into this one file, it will become rather big. Smaller
files are probably easier to work with. We could still keep small classes like
`TreeElementIDLibrary` in the general file, don't mind really, but this creates
separate files for that too.
