File headers: SPDX License migration

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

source/blender/blenlib/tests/BLI_memory_utils_test.cc

@@ -1,4 +1,4 @@
-/* Apache License, Version 2.0 */
+/* SPDX-License-Identifier: Apache-2.0 */
 
 #include "BLI_math_vec_types.hh"
 #include "BLI_memory_utils.hh"