Revise mac app

[jasonrclark]

Large set of changes to get the Mac app support more lined up with what I did for Windows (and where I'm aiming all the apps to go). Changes include:

• Renaming JarApp to MacApp to be more accurate
• Running off the BaseApp class more effectively
• Rely on the newly minted https://github.com/shoes/mac-app-templates for template download, and intro a revised template
  • The new template simplifies some things, for instance reducing nesting, always making the JAR file a single name, and using a script instead of the Java launcher to get things started (which will buy us leverage to download our own JRE if java's missing, just like the windows template).

Still some template revision to do, but that's outside of furoshiki or shoes... should just be in the mac-app-templates repo.

Coordinating PR in Shoes at https://github.com/shoes/shoes4/pull/1430