Add Support to Comply with AppImageHub

[FoggyMtnDrifter]

If you're going to encourage users to consider publishing to AppImageHub, you may want to implement the features required to publish there. When I submitted a PR, the CI bot rejected it with the following information:

[AppImageHub]
# Dear upstream developer, please include update information in your AppImage
# (e.g., with appimagetool -u) so that users can easily update the AppImage
X-AppImage-Signature=[don't know]: invalid packet (ctb=0a) no signature found the signature could not be verified. Please remember that the signature file (.sig or .asc) should be the first file given on the command line. 
X-AppImage-Type=2
X-AppImage-Architecture=x86_64
ls: cannot access 'database/Infinity/screenshot.png': No such file or directory

The two failures are related to X-AppImage-Signature and a screenshot of the application in database/Infinity/screenshot.png. Personally for the screenshot, I would just grab a screenshot of the webpage while building and put it in the requested directory.

Just wanna help improve your awesome tool!