Draft: plug-ins: macos scanner plugin

This is based on https://github.com/SimulPiscator/GimpScan.

It currently compiles and will run, but it does not work.

I'm not sure how to fix it.

To do:

• Make it work, which seems to be turning it into a cocoa application which sits inside of gimp, and creates its own gui. I don't know how to do this properly. The remaining blocker appears to be getting the MainMenu.xib to load, however I was not able to get the original project to run either so there may be deeper problems.
• Ensure this only is compiled on MacOS (this should be easy)
• Get installation working properly. This requires copying the Info.plist to the destination directory. However, shouldn't be attempted until the first solution/situation is solved.

This was an attempt to deliver on #5120.