appimaged: implement appimage_shall_not_be_integrated

[TheAssassin]

CC @probonopd

appimaged should respect the new X-AppImage-Integrate=false key that has been implemented by #750.

[TheAssassin]

@probonopd I would still like to know whether you want this in appimaged or not. It's a small patch, but it changes the way appimaged works, therefore I ask you first.

[probonopd]

Yes, I'd like this in appimaged. Overall, I'd like appimaged and similar solutions to share as much code and behavior as possible. If we can't entirely merge them.

[TheAssassin]

Want to give this a try? It's a very small patch, and it'd provide you a first level view on our API design.

[probonopd]

I'd highly appreciate a PR @TheAssassin

[TheAssassin]

Please don't expect me to do it anytime soon. I have tons of other things to do at the moment.

[TheAssassin]

Migrated to AppImageCommunity/appimaged#20.