Support 'app-image' as package type

[jjohannes]

Some users also would like to ship "portable versions" of their app (i.e. package without installer). These can be built using the app-image package type, which we already use as intermediate result. Users should be able to explicitly request this package type for a target and it should be part of the task output (currently it is put into a temp directory).

Required for: JabRef/jabref#13401

[jjohannes]

Initial implementation done on preview branch.

[koppor]

Interesting side-track: --win-console and msi. I just note it down here that it noted down here in the repo.

Error: Option [--win-console] is not valid with type [msi]