Bugfix/windows build support whitespace in path

[retrobit]

From issue: https://github.com/Gemba/skyscraper/issues/11

If user home or build path contained white space, build was failing on Windows

For some reason, shell_quote() qmake function when calling powershell -Command md -Force ... was not treated as a single string with white space, but rather multiple strings separated by white space

• Replaced outer single quotes (which PowerShell interprets enclosed string as verbatim) containing command with double-quotes (which PowerShell interprets enclosed string as expandable)
• Replaced $$shell_path() surrounding path in PowerShell md command with single quotes
• Added qmake functions for native shell handling of quotes and path for file parameter that was missing it
• Updated docs on building on Windows

Attached are text output and screenshot artifacts of build failure (before) and build success (after)

build failure.txt build success.txt