text: Title: Refactor Config handling
  Body: * consistent keys (uppercase with underscore), but accept lowercase with spaces, too, for legacy compatibility
  * central handling in Python
  * allow non-ASCII characters
  * cross-platform compatibility (currently, config files created on macOS won't be read on Windows because of line endings)
  * make all settings available for `_WIN` and `_MAC`. `PYTHONPATH`, for example, doesn't support this yet.
  * 2 empty lines at the end of the config file causes "Could not activate Python COM server, hr = -2147221164 1000" after the 2min timeout
  * https://github.com/xlwings/xlwings/issues/574
  * https://github.com/xlwings/xlwings/issues/550