Add variable `CMAKE_CODEBLOCKS_EXCLUDE_EXTERNAL_FILES` to optionally
exclude files from outside the project root from the project file
written by the CodeBlocks extra generator.  This optionally restores
logic that had been removed by commit v2.8.3~40^2 (CodeBlocks Generator:
Do not omit files in the project file listing, 2010-10-05) in response
to QTCREATORBUG-2250.

Issue: #12110
Fixes: #17188