[jpackage] Run custom action/script during installation

[Tobias Diez]

Hi all,

I've been experimenting with the recent build of jpackage and in general it
is working quite well. Thanks for your work!

One feature that I was strongly missing (or at least couldn't find anything
in the docs) is to run an own script during installation. For example, I
need to register components in the registry (for Windows), create files
(e.g. in /Library on Mac or /usr/lib/ on Linux) and copy custom resources to
the installation directory.

It would be nice to add such a feature because I guess most installers need
to do more than copying a few files into a given folder.

Best regards
Tobias