2024-01 octapp-bundler nicification

[apjanke]

Some little things to make octapp-bundler nicer and easier to use.

For work on #243 Octave 8.x and macOS 11+.

TODO

• Figure out why a call on a fresh app build is exiting after the brew install octave-octapp@x.y.z stage.
  • Probably an errexit after brew returning nonzero?
• Do the build munging in a separate ./munging dir, and then copy to ./build, so the existence of the file there means it's all munged, and not in-progress.
• Put intermediate DMGs somewhere else too, so a DMG in ./build means it's finished.
  • or maybe use ./build for intermediates, and have a dist/ or target/ dir for finals.

[apjanke]

Renamed this from "2024-01 octapp-bundler nicification" to "2024-Q1 ..." because I'm not that ambitious, and obviously already missed that ETA.