…and if so, is it possible to choose the application update channel in AppCenter?
Ubuntu 18.04 will support these features, will elementary OS include it in 5.0 version?
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Snaps are not supported in the AppCenter but you can install Snaptastic. Snaptastic will manage Snaps, including updates, installation and removal. Snaptastic is found in the AppCenter.
Software sources are not by default managed by AppCenter but you can install Software and Sources in the AppCenter. This is the other half of ubuntu software center that allows you to manage sources, change update frequency, additional drivers and more.
Software and Sources
I don't think snap packages will come to eOS 5.0 because it would pretty much obsolete the eOS App Center. However, if you know which software you'd like to have, you can simply install snap and download your software (Spotify is one I can think of) via
apt install snapd
You install things with:
and add the package name at the end like snap install spotify for instance.