Clarify the UX for Independent Container Updates

[almet]

For #1006, the UX story is not yet fully defined.

Currently, the UX is that you can opt-in so that Dangerzone checks for new updates on your behalf. When a new release is detected, a dot is appended to the hamburger menu, and you're notified that there is an update.

Here is a round of questions:

• Should we do the same thing for independent container updates?
• To which level of detail / complexity do we want to expose our users?
• Should we merge the two "updates" settings together in the UI?

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I have my takes on these, but I'm curious what others think about it, especially @harrislapiroff