Allow `appify.sh` to select a single platform and architecture?

[marktsuchida]

If an app is shipped as separate builds for each OS/arch, the build system is often run per-arch as a whole (as opposed to a single build system producing artifacts for multiple archs). In this context, it would be nice if Jaunch could be installed into the app, but only for a single architecture.

(There may be cases where a limited set of multiple OS/archs are desired, but this can hopefully be addressed by multiple invocations of appfy.sh.)

For macOS in particular, I think it is often nice to ship either the universal binaries or (one or both) thin binaries, but not both. And if selecting just one of universal/x64/arm64, the setting of the bundle executable could reflect the choice.

Also, on Windows, perhaps some apps will just want to have the -gui launcher without the -console one.

Perhaps a flag --platform= where the value is macos, macos-x64, windows-x64-gui, linux-arm64 etc.?

[marktsuchida]

There might be the additional desire to install a .app bundle but not the bare executables, or whether or not to include the launcher .sh and .bat.

All of this can be worked around by removing the undesired executables (and patching Info.plist if applicable), of course.

[ctrueden]

I took a quick look at doing this, but I think it will necessitate a major restructuring of the appify scripts, so I'm going to hold off for now.