Consider adding override settings for build setting info

[dempseyatgithub]

There may be cases where BSE needs to override any settings found in Xcode. A few current examples:

• The SWIFT_VERSION setting has a name but an empty description. I believe any backstop would be overridden by the blank description.

• The Xcode naming of SWIFT_COMPILATION_MODE is just 'Compilation Mode'. A more clear name in an xcconfig file would be 'Swift Compilation Mode'. An override could substitute that in.

• Markdown or Markdown-ish formatting It looks like some build settings in Xcode, like SWIFT_COMPILATION_MODE are starting to use Markdown for the description string. I'd prefer not to have to add a whole Markdown parser to BSE for that purpose. An override setting would give an opportunity to format the string so it looks better in an xcconfig file

Testing: Possibly store the original string value along with the overridden value, so a test can be written to alert if the original string value changes in Xcode, in case the description changes or is improved.