Add script to generate macOS icon file. It does not attempt to integrate with the current build process as the macOS icon file is mostly static.
`libsvg` was chosen because of its low dependency count over other solutions that use backends like Chromium, PhontomJS, etc. ImageMagick's `convert` was tested but resulting PNGs where not satisfactory and would have required an overcomplicated script having to calculate the `density` argument.
* Has a dependency on librsvg (can be installed via Brew)
* Uses SVG file as input
* Can be integrated in later to build process if desired
Closes#7455
