Add missing appimagetool command line parameters

[Korne127]

There are some missing command line parameters in the original appimagetool that are required in some use cases and still missing. To ensure compatibility and to be able to replace the original appimagetool also in these use-cases, those CLI parameters should be added.

The most important one I found is --updateinformation. It is currently not possible to manually add an update information string in appimagetool, even though the underlying mkappimage supports it (see this). Instead, the update information is only set if appimagetool is executed in a CLI with no way to disable that. This means that people can't set an own update information string, e.g. to use their own server or to execute it locally.

Additionally, there is the --sign parameter. It seems like signing is now done automatically, which is understandable, but then there should still be a --no-sign parameter to be able to disable it.
Also, signing depends on the public key being stored in the root directory of the git repository and the encrypted private key being stored in the current working directory and an undocumented environment variable called $super_secret_password; apparently tailored to Travis CI. This should probably be documented and explained somewhere. Additionally, it should probably be possible to just pass the id of the public key, similar to how the linuxdeploy AppImage plugin works for a more convenient mainstream usage.

There might be other command line parameters that the original appimagetool supports and this rewrite doesn't (yet), but these are the two I originally noticed. However, it would be great imo to also support the features the original appimagetool supports.

[Korne127]

I added the current state of embedding updating information and signing the AppImage to the documentation (WIP).

[probonopd]

Hi @Korne127, our experience with https://github.com/AppImage/AppImageKit over the last 10 or so years has been that few develoepers actually used the signing and updating features correctly; possibly because they were not straightforward enough to use. So we'd like to make this as automatic as possible. That's at least the idea.

[Korne127]

Thanks for the answer :)

So we'd like to make this as automatic as possible

I don't think that this itself is a bad idea. I can understand the advantages of that. But I definitely think that it should still be possible to manually override that, e.g. opting out of signing or manually giving an update information string, as it's possible with the original appimagetool.

Besides that, I do think that a clear and concise documentation explaining the systems heavily improves the ease of use of such features and helps developers to use them correctly (which is partly why I've been working on this for the last few months).